Palladium-containing particles, method and apparatus of manufacturing palladium-containing devices made therefrom

ABSTRACT

Provided are palladium-containing powders and a method and apparatus for manufacturing the palladium-containing particles of high quality, of a small size and narrow size distribution. An aerosol is generated from liquid feed and sent to a furnace, where liquid in droplets in the aerosol is vaporized to permit formation of the desired particles, which are then collected in a particle collector. The aerosol generation involves preparation of a high quality aerosol, with a narrow droplet size distribution, with close control over droplet size and with a high droplet loading suitable for commercial applications. Powders may have high resistance to oxidation of palladium. Multi-phase particles are provided including a palladium-containing metallic phase and a second phase that is dielectric. Electronic components are provided manufacturable using the powders.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/790,958 filed Mar. 2, 2004, now U.S. Pat. No. 7,172,663which is a divisional application of U.S. patent application Ser. No.09/668,441 filed Sep. 22, 2000, now U.S. Pat. No. 6,699,304 which is acontinuation of U.S. patent application Ser. No. 09/028,751 filed Feb.24, 1998, now U.S. Pat. No. 6,159,267 which claims priority toProvisional Patent Application Nos. 60/039,450 and 60/038,258 both ofwhich were filed Feb. 24, 1997. Each of these applications is herebyincorporated by reference in its entirety as if set forth herein infull.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH/DEVELOPMENT

This invention was made with Government support under Contract No.N00014-95-C0278 and N00014-96-C0395 awarded by the Office of NavalResearch. The Government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention involves an aerosol method for producing apalladium-containing particulate product, palladium-containingparticulate products producible by the method, uses of the powders inthe manufacture of electronic products, and electronic products somanufactured.

BACKGROUND OF THE INVENTION

Palladium is widely used in the manufacture of electrically conductivethick films for microelectronic devices. A trend continues, however, toproduce ever smaller particles for thick film applications. Generally,desirable features in such small particles include a small particlesize; a narrow particle size distribution; a dense, spherical particlemorphology; and a crystalline grain structure. Existing technology formanufacturing palladium-containing particles could, however, be improvedwith respect to attaining all, or substantially all, of these desiredfeatures for particles used in thick film applications. Improvements inthe particles could result in significant performance advantages and/orcost savings when used to make electronic or other products.

One method that has been used to make small particles is to precipitatethe particles from a liquid medium. Such liquid precipitation techniquesare often difficult to control to produce particles with the desiredcharacteristics. It is especially difficult by the liquid precipitationroute to obtain particles having dense, spherical particle morphologyand with good crystallinity.

Aerosol methods have also been used to make small palladium particles.One aerosol method for making small particles is spray pyrolysis, inwhich an aerosol spray is generated and then converted in a reactor tothe desired particles. Spray pyrolysis systems involving palladium have,however, been mostly experimental, and unsuitable for commercialparticle production. Furthermore, control of particle size distributionis a concern with spray pyrolysis. Also, spray pyrolysis systems areoften inefficient in the use of carrier gasses that suspend and carryliquid droplets of the aerosol. This inefficiency is a majorconsideration for commercial applications of spray pyrolysis systems.

Additionally, palladium is easily oxidizable and has a tendency tooxidize during the manufacture of electronic devices. The oxidation ofpalladium during manufacture of electronic devices is problematicbecause volume expansion that accompanies oxidation can cause filmcracking and delamination. It has been proposed that the susceptibilityof palladium particles to oxidation is reduced by the addition of asmall amount of an alkaline earth metal and by making the palladiumparticles as single-crystal particles to eliminate diffusion of oxygenalong grain boundaries. Even with an alkaline earth additive, however,oxidation of palladium during the manufacture of microelectronic devicesis still a significant problem and the cost of making single-crystalparticles is high because of the high processing temperatures and longresidence times required.

There is a need for improved palladium-containing powders, for improvedmethods of manufacturing palladium-containing powders and for improvedproducts incorporating or made using improved palladium-containingpowders.

SUMMARY OF THE INVENTION

The present invention provides powders of high quality, micro-sizepalladium-containing particles of a variety of compositions and particlemorphologies, and with carefully controlled particle size and sizedistribution, and an aerosol method for producing the particles. Theparticles are useful for making a variety of products.

Through careful control of the aerosol generation, and in combinationwith other features of the present invention concerning manufacture ofpalladium-containing powders, the process of the present inventionpermits preparation of very high quality powders of palladium-containingparticles that preferably have a weight average size of typically about0.1 micron to about 4 microns in size, and for many applications fromabout 0.2 micron to about 0.8 micron in size. The powders also have anarrow size distribution such that typically at least about 90 weightpercent of the particles being smaller than about twice the weightaverage size of the particles. Furthermore, typically less than about 10weight percent, and preferably less than about 5 weight percent, of theparticles are smaller than about one-half the weight average size of theparticles.

The invention includes both single-phase and multi-phase, or composite,particles useful for a variety of product applications, including foruse as catalysts and in the preparation of thick film pasteformulations, such as are used for depositing palladium-containing filmsduring manufacture of various electronic and other products. Multi-phasematerials may be in a variety of morphological forms, such as anintimate mixture of two or more phases or with one phase forming asurface coating over a core including another phase.

One preferred class of multi-phase particles includes a metallicpalladium-containing phase and a nonmetallic phase, which frequentlyincludes a ceramic material. The nonmetallic phase could be in the formof a coating around a core of the metallic phase, in the form of smalldomains dispersed in a matrix of the metallic phase, or in some otherform. A variety of ceramic and other materials can be used to effect avariety of beneficial modifications to particle properties, such as amodification of densification/sintering properties for improvedcompatibility and bonding with ceramic dielectric layers in electronicdevices, and especially when the palladium-containing powder is used tomake internal electrodes in multi-layer capacitors. An important use ofmulti-phase particles is to reduce film shrinkage during firing in themanufacture of electronic devices. One preferred group of ceramicmaterials for use in the multi-phase particles are titanates, as arefrequently used in dielectric layers of multi-layer capacitors. Anotherpreferred group of ceramic materials for use in multi-phase particlesincludes silica, alumina, titania and zirconia, and especially silicaand alumina.

The palladium-containing powders may also be made with surprisingly highresistance to palladium oxidation, providing a significant advantageduring the manufacture of thick film electronic products by reducingvolume expansion during firing. This is particularly important in themanufacture of multi-layer capacitors, multi-chip modules and othercofired products where volume expansions due to palladium oxidation canresult in significant film cracking and delaminations. The highoxidation resistance of the particles is particularly surprising becausethe resistance to palladium oxidation may be obtained without the use ofalkaline earth metal additives. This is even more surprising because theparticles may be made to exhibit good oxidation resistance even when theparticles are polycrystalline. This is particularly advantageous becauseparticles of high oxidation resistance may be made without thesignificant operating expense required to make single crystal particles.In that regard, the maximum average stream temperature in the pyrolysisfurnace should typically be in a range of from about 900° C. to about1300° C., although other ranges may be more preferred in somecircumstances. Furthermore, although the powders exhibit high resistanceof palladium to oxidation without any additives, it has also been foundwith the present invention that oxidation resistance may be furtherimproved by the addition of small quantities of tin to the particles.

The palladium in the powder is typically in a metallic phase, whether ina single phase or multi-phase particles. In one embodiment of theinvention, the particles of the powder includes high quality palladiumalloys, and especially alloys with silver. It has been found that thequality of the alloy is highly dependent upon processing conditions.When preparing particles including a palladium/silver alloy, the maximumaverage stream temperatures in a furnace reactor should be in the rangeof from about 900° C. to about 1200° C., with even narrower temperatureranges being more preferred for better control of alloy quality. Asurprising result of the high quality alloy is that the palladium in thealloy shows a remarkable resistance to oxidation, which is particularlyadvantageous in many applications for the manufacture of electronicproducts.

The process of the present invention for making the palladium-containingparticles involves processing of a high quality aerosol including apalladium-containing precursor. The aerosol includes droplets ofcontrolled size suspended in and carried by a carrier gas. In a thermalreactor, typically a furnace reactor, the liquid of the droplets isvaporized, permitting formation of the desired particles in an aerosolstate. According to one embodiment of the present invention, an aerosolat a high droplet loading and at a high volumetric flow rate is fed to areactor, where particles are formed. In addition to the high dropletloading and high volumetric flow rate, the aerosol also includes anarrow size distribution of droplets such that the particles exiting thereactor also have a narrow size distribution, with preferably at leastabout 75 weight percent, and more preferably at least 90 weight percent,of the particles being smaller than about twice the weight averageparticle size.

With the process, and accompanying apparatus, of the present invention,it is possible to produce high quality palladium-containing powders at ahigh production rate using spray pyrolysis. This represents asignificant advancement relative to the small laboratory-scale processescurrently used.

An important aspect of the method of the present invention is aerosolgeneration, in which a high quality aerosol is produced having acontrolled droplet size and narrow droplet size distribution, but at ahigh volumetric flow rate and with high droplet loading. An ultrasonicgenerator design is provided for generation of the high quality aerosolat a high generation rate.

The aerosol generation is particularly advantageous for producing highaerosol flow rates with droplets having a weight average size of fromabout 1 micron to about 5 microns, preferably with no greater than about30 weight percent of the droplets being larger than about two times theaverage droplet size.

High quality aerosol production is accomplished also with high dropletloading in the aerosol. The droplet loading is preferably greater thanabout 5×10⁶ droplets per cubic centimeter of the aerosol. Furthermore,the aerosol typically includes greater than about 0.083 milliliters ofdroplets in the aerosol per liter of carrier gas in the aerosol. Thishigh droplet loading is also accomplished at a high aerosol productionrate, which is typically at a rate of greater than about 25 millilitersof droplets of liquid feed per hour per ultrasonic transducer. Totalaerosol flow rates are typically larger than about 0.5 liter per hour ofliquid droplets at the high droplet loading and with the narrow dropletsize distribution.

Aerosol generation for particle manufacture of the present invention isbelieved to represent a significant improvement for powder manufacturerelative to current powder manufacture operations, which are mainly forexperimental purposes. These laboratory-scale processes typically useaerosols at only low rates and normally without a high aerosol density.With the aerosol generator of the present invention, however, high ratesof droplet production are possible with efficient use of carrier gas. Inone embodiment, the aerosol generator includes a plurality of ultrasonictransducers underlying a reservoir of liquid feed that is ultrasonicallyenergized during operation. The aerosol generator includes multiple gasdelivery outlets, or ports, for delivery of carrier gas to differentportions of a liquid feed reservoir, so that droplets generated from thedifferent portions of the reservoir are efficiently swept away to formthe aerosol. A preferred embodiment includes at least one gas deliveryoutlet per ultrasonic transducer.

The process and the apparatus of the present invention are also capableof producing palladium-containing powder at a high rate without highlosses of palladium in the system. This is accomplished through carefulcontrol of process equipment and operating parameters in a manner toreduce system residence times to inhibit high production losses. Animportant aspect of the process of the present invention is operation ofa pyrolysis furnace at high flow rates and in a manner to reduce thepotential for losses of palladium. When operating at the high Reynoldsnumbers typically encountered in the furnace with the present invention,it is important to carefully control the furnace temperature andtemperature profile. For example, it is important to operate the furnaceso that both the average maximum stream temperature and the maximumfurnace wall temperature are low enough to avoid an undesirablevolatilization of components.

In a further aspect of the process and apparatus of the invention, theparticles may be advantageously cooled for collection in a manner toreduce potential for palladium losses. The particle cooling mayadvantageously be accomplished with a very short residence time byradial feed of a quench gas into a cooling conduit through which theparticle-containing aerosol stream flows. In this manner, a cool gasbuffer is developed around the inner walls of the cooling conduit,thereby reducing thermophoretic losses of particles during cooling.

Also, the particle manufacturing process of the present invention isversatile and may be adapted for preparation of a variety ofpalladium-containing particulate materials for a variety ofapplications. In that regard, one embodiment of the present inventionincludes concentration of the aerosol by at least a factor of about two,and more preferably by a factor of greater than about five, beforeintroduction of the aerosol into the reactor. In this manner,substantial savings may be obtained through lower heating requirementsin the reactor, lower cooling requirements for product streams from thereactor and smaller process equipment requirements.

In another embodiment, the process of the present invention involvesclassification by size of the droplets in the aerosol prior tointroduction into the pyrolysis furnace. Preferably, droplets largerthan about three times the average droplet size are removed, and evenmore preferably droplets larger than about two times the average dropletsize are removed.

In yet another embodiment of the present invention, the particles aremodified following manufacture, while still dispersed in an aerosolstream, prior to particle collection. In one aspect, the particles maybe subjected to a coating following manufacture, such as bygas-to-particle conversion processes. Preferred coating processesinclude chemical vapor deposition and physical vapor deposition. In afurther aspect, the particle modification may involve a structuralmodification, such as a post manufacture anneal to improve crystallinityor to alter particle morphology.

The present invention also includes thick film paste formulationsincluding the palladium-containing particles of the present inventionand processes of manufacturing films from paste formulations. Alsoincluded in the present invention are methods for making electronic andother products using the palladium-containing particles and the productsso manufactured. The present invention provides a variety of productsmade using powder of the present invention. These products includeelectronic devices, such as multi-layer capacitors and multi-chipmodules, and other products, such as catalysts. Also, the powders areparticularly useful for making high definition patterned circuit lineswith a close line spacing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process block diagram showing one embodiment of the processof the present invention.

FIG. 2 is a side view of a furnace and showing one embodiment of thepresent invention for sealing the end of a furnace tube.

FIG. 3 is a view of the side of an end cap that faces away from thefurnace shown in FIG. 2.

FIG. 4 is a view of the side of an end cap that faces toward the furnaceshown in FIG. 2.

FIG. 5 is a side view in cross section of one embodiment of aerosolgenerator of the present invention.

FIG. 6 is a top view of a transducer mounting plate showing a 49transducer array for use in an aerosol generator of the presentinvention.

FIG. 7 is a top view of a transducer mounting plate for a 400 transducerarray for use in an ultrasonic generator of the present invention.

FIG. 8 is a side view of the transducer mounting plate shown in FIG. 7.

FIG. 9 is a partial side view showing the profile of a single transducermounting receptacle of the transducer mounting plate shown in FIG. 7.

FIG. 10 is a partial side view in cross-section showing an alternativeembodiment for mounting an ultrasonic transducer.

FIG. 11 is a top view of a bottom retaining plate for retaining aseparator for use in an aerosol generator of the present invention.

FIG. 12 is a top view of a liquid feed box having a bottom retainingplate to assist in retaining a separator for use in an aerosol generatorof the present invention.

FIG. 13 is a side view of the liquid feed box shown in FIG. 8.

FIG. 14 is a side view of a gas tube for delivering gas within anaerosol generator of the present invention.

FIG. 15 shows a partial top view of gas tubes positioned in a liquidfeed box for distributing gas relative to ultrasonic transducerpositions for use in an aerosol generator of the present invention.

FIG. 16 shows one embodiment for a gas distribution configuration forthe aerosol generator of the present invention.

FIG. 17 shows another embodiment for a gas distribution configurationfor the aerosol generator of the present invention.

FIG. 18 is a top view of one embodiment of a gas distribution plate/gastube assembly of the aerosol generator of the present invention.

FIG. 19 is a side view of one embodiment of the gas distributionplate/gas tube assembly shown in FIG. 18.

FIG. 20 shows one embodiment for orienting a transducer in the aerosolgenerator of the present invention.

FIG. 21 is a top view of a gas manifold for distributing gas within anaerosol generator of the present invention.

FIG. 22 is a side view of the gas manifold shown in FIG. 21.

FIG. 23 is a top view of a generator lid of a hood design for use in anaerosol generator of the present invention.

FIG. 24 is a side view of the generator lid shown in FIG. 23.

FIG. 25 is a process block diagram of one embodiment in the presentinvention including an aerosol concentrator.

FIG. 26 is a top view in cross section of a virtual impactor that may beused for concentrating an aerosol according to the present invention.

FIG. 27 is a front view of an upstream plate assembly of the virtualimpactor shown in FIG. 26.

FIG. 28 is a top view of the upstream plate assembly shown in FIG. 27.

FIG. 29 is a side view of the upstream plate assembly shown in FIG. 27.

FIG. 30 is a front view of a downstream plate assembly of the virtualimpactor shown in FIG. 26.

FIG. 31 is a top view of the downstream plate assembly shown in FIG. 30.

FIG. 32 is a side view of the downstream plate assembly shown in FIG.30.

FIG. 33 is a process block diagram of one embodiment of the process ofthe present invention including a droplet classifier.

FIG. 34 is a top view in cross section of an impactor of the presentinvention for use in classifying an aerosol.

FIG. 35 is a front view of a flow control plate of the impactor shown inFIG. 34.

FIG. 36 is a front view of a mounting plate of the impactor shown inFIG. 34.

FIG. 37 is a front view of an impactor plate assembly of the impactorshown in FIG. 34.

FIG. 38 is a side view of the impactor plate assembly shown in FIG. 37.

FIG. 39 shows a side view in cross section of a virtual impactor incombination with an impactor of the present invention for concentratingand classifying droplets in an aerosol.

FIG. 40 is a process block diagram of one embodiment of the presentinvention including a particle cooler.

FIG. 41 is a top view of a gas quench cooler of the present invention.

FIG. 42 is an end view of the gas quench cooler shown in FIG. 41.

FIG. 43 is a side view of a perforated conduit of the quench coolershown in FIG. 41.

FIG. 44 is a side view showing one embodiment of a gas quench cooler ofthe present invention connected with a cyclone.

FIG. 45 is a process block diagram of one embodiment of the presentinvention including a particle coater.

FIG. 46 is a block diagram of one embodiment of the present inventionincluding a particle modifier.

FIG. 47 shows cross sections of various particle morphologies of somecomposite particles manufacturable according to the present invention.

FIG. 48 shows a side view of one embodiment of apparatus of the presentinvention including an aerosol generator, an aerosol concentrator, adroplet classifier, a furnace, a particle cooler, and a particlecollector.

FIG. 49 is a block diagram of one embodiment of the process of thepresent invention including the addition of a dry gas between theaerosol generator and the furnace.

FIG. 50 is a perspective view with partial cutaway of a multi-layercapacitor of the present invention.

FIG. 51 is a partial cross-sectional view of a stacked layer structureof one embodiment of a multi-layer capacitor of the present invention.

FIG. 52 is a partial cross-sectional view of a stacked layer structureof another embodiment of a multi-layer capacitor of the presentinvention.

FIG. 53 is a perspective view with a cut away end showing a multi-chipmodule with various electrical interconnections made using powder of thepresent invention.

FIG. 54 is a partial top view showing a flat panel display includingaddress electrodes made using powder of the present invention.

FIG. 55 is a partial side view in cross-section showing a DC plasmadisplay including an address electrode made using powder of the presentinvention.

FIG. 56 is a partial side view in cross-section showing an AC plasmadisplay including an address electrode made using powder of the presentinvention.

FIG. 57 is a photomicrograph showing palladium/silver alloy particlesmanufactured at a reactor temperature of 900° C.

FIG. 58 is a photomicrograph showing palladium/silver alloy particlesmanufactured at a reactor temperature of 1000° C.

FIG. 59 is a photomicrograph showing palladium/silver alloy particlesmanufactured at a reactor temperature of 1400° C.

FIG. 60 is a photomicrograph showing palladium/silica multi-phaseparticles.

FIG. 61 is a SEM photomicrograph showing a composite particle including20 weight percent barium titanate and 80 weight percent of a 30:70 Pd:Agalloy made at a reactor temperature of 1000° C.

FIG. 62 is a TEM photomicrograph showing composite particles including20 weight percent barium titanate and 80 weight percent of a 30:70 Pd:Agalloy made at a reactor temperature of 1000° C.

FIG. 63 is a TEM photomicrograph showing composite particles including 5weight percent barium titanate and 95 weight percent of a 30:70 Pd:Agalloy made at a reactor temperature of 1000° C.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, the present invention provides a method for preparing aparticulate product. A feed of liquid-containing, flowable medium,including at least one palladium-containing precursor for the desiredparticulate product, is converted to aerosol form, with droplets of themedium being dispersed in and suspended by a carrier gas. Liquid fromthe droplets in the aerosol is then removed to permit formation in adispersed state of the desired particles. Typically, the feed precursoris pyrolyzed in a furnace to make the particles. In one embodiment, theparticles are subjected, while still in a dispersed state, tocompositional or structural modification, if desired. Compositionalmodification may include, for example, coating the particles. Structuralmodification may include, for example, crystallization,recrystallization or morphological alteration of the particles. The termpowder is often used herein to refer to the particulate product of thepresent invention. The use of the term powder does not indicate,however, that the particulate product must be dry or in any particularenvironment. Although the particulate product is typically manufacturedin a dry state, the particulate product may, after manufacture, beplaced in a wet environment, such as in a slurry.

The process of the present invention is particularly well suited for theproduction of palladium-containing particulate products of finelydivided particles having a weight average size, for most applications,in a range having a lower limit of about 0.1 micron, preferably about0.3 micron, more preferably (for some applications) about 0.5 micron andeven more preferably (for some applications) about 0.8 micron; andhaving an upper limit of about 4 microns, preferably about 3 microns,more preferably about 2.5 microns, even more preferably about 2 micronsand most preferably about 1 micron (for some applications). Aparticularly preferred range for many applications is a weight averagesize of from about 0.5 micron to about 3 microns, and more particularlyfrom about 0.5 micron to about 2 microns. For some applications,however, other weight average particle sizes may be particularlypreferred. For example, for making internal electrodes of multi-layercapacitors, a weight average particles size of from about 0.2 micron toabout 0.8 micron is preferred, and particularly from about 0.3 micron toabout 0.6 micron.

In addition to making particles within a desired range of weight averageparticle size, with the present invention the particles may be producedwith a desirably narrow size distribution, thereby providing sizeuniformity that is desired for many applications.

In addition to control over particle size and size distribution, themethod of the present invention provides significant flexibility forproducing particles of varying composition, crystallinity andmorphology. For example, the present invention may be used to producehomogeneous particles involving only a single phase or multi-phaseparticles including multiple phases. In the case of multi-phaseparticles, the phases may be present in a variety of morphologies. Forexample, one phase may be uniformly dispersed throughout a matrix ofanother phase. Alternatively, one phase may form an interior core whileanother phase forms a coating that surrounds the core. Othermorphologies are also possible, as discussed more fully below.

Referring now to FIG. 1, one embodiment of the process of the presentinvention is described. A liquid feed 102, including at least onepalladium-containing precursor for the desired particles, and a carriergas 104 are fed to an aerosol generator 106 where an aerosol 108 isproduced. The aerosol 108 is then fed to a furnace 110 where liquid inthe aerosol 108 is removed to produce palladium-containing particles 112that are dispersed in and suspended by gas exiting the furnace 110. Theparticles 112 are then collected in a particle collector 114 to producea particulate product 116.

As used herein, the liquid feed 102 is a feed that includes one or moreflowable liquids as the major constituent(s), such that the feed is aflowable medium. The liquid feed 102 need not comprise only liquidconstituents. The liquid feed 102 may comprise only constituents in oneor more liquid phase, or it may also include particulate materialsuspended in a liquid phase. The liquid feed 102 must, however, becapable of being atomized to form droplets of sufficiently small sizefor preparation of the aerosol 108. Therefore, if the liquid feed 102includes suspended particles, those particles should be relatively smallin relation to the size of droplets in the aerosol 108. Such suspendedparticles should typically be smaller than about 1 micron in size,preferably smaller than about 0.5 micron in size, and more preferablysmaller than about 0.3 micron in size and most preferably smaller thanabout 0.1 micron in size. Most preferably, the suspended particlesshould be able to form a colloid. The suspended particles could befinely divided particles, or could be agglomerate masses comprised ofagglomerated smaller primary particles. For example, 0.5 micronparticles could be agglomerates of nanometer-sized primary particles.When the liquid feed 102 includes suspended particles, the particlestypically comprise no greater than about 25 to 50 weight percent of theliquid feed.

As noted, the liquid feed 102 includes at least one precursor forpreparation of the particles 112. The precursor may be a substance ineither a liquid or solid phase of the liquid feed 102. Frequently, theprecursor will be a material, such as a salt, dissolved in a liquidsolvent of the liquid feed 102. The precursor may undergo one or morechemical reactions in the furnace 110 to assist in production of theparticles 112. Alternatively, the precursor material may contribute toformation of the particles 112 without undergoing chemical reaction.This could be the case, for example, when the liquid feed 102 includes,as a precursor material, suspended particles that are not chemicallymodified in the furnace 110. In any event, the particles 112 comprise atleast one component originally contributed by the precursor.

The liquid feed 102 may include multiple precursor materials, which maybe present together in a single phase or separately in multiple phases.For example, the liquid feed 102 may include multiple precursors insolution in a single liquid vehicle. Alternatively, one precursormaterial could be in a solid particulate phase and a second precursormaterial could be in a liquid phase. Also, one precursor material couldbe in one liquid phase and a second precursor material could be in asecond liquid phase, such as could be the case when the liquid feed 102comprises an emulsion. Different components contributed by differentprecursors may be present in the particles together in a single materialphase, or the different components may be present in different materialphases when the particles 112 are composites of multiple phases.Preferred palladium-containing precursors include palladium salts, suchas nitrate, chloride and acetate salts, which are soluble in aqueousliquids.

When the liquid feed 102 includes a soluble precursor, the precursorsolution should be unsaturated to avoid the formation of precipitates.Solutions of salts will typically be used in concentrations in a rangeto provide a solution including from about 1 to about 50 weight percentsolute. Most often, the liquid feed will include a solution of apalladium salt with from about 2 weight percent to about 10 weightpercent palladium. Preferably the solvent is aqueous-based for ease ofoperation, although other solvents, such as toluene or other organicsolvents, may be desirable for specific materials. The use of organicsolvents, however, can sometimes lead to undesirable carboncontamination in the particles. The pH of the aqueous-based solutionscan be adjusted to alter the solubility characteristics of the precursoror precursors in the solution.

The carrier gas 104 may comprise any gaseous medium in which dropletsproduced from the liquid feed 102 may be dispersed in aerosol form.Also, the carrier gas 104 may be inert, in that the carrier gas 104 doesnot participate in formation of the particles 112. Alternatively, thecarrier gas may have one or more active component(s) that contribute toformation of the particles 112. In that regard, the carrier gas mayinclude one or more reactive components that react in the furnace 110 tocontribute to formation of the particles 112. Although air may be usedas the carrier gas, when making the palladium-containing particles ofthe present invention, nitrogen or another inert gas, such as argon, ispreferred.

The aerosol generator 106 atomizes the liquid feed 102 to form dropletsin a manner to permit the carrier gas 104 to sweep the droplets away toform the aerosol 108. The droplets comprise liquid from the liquid feed102. The droplets may, however, also include nonliquid material, such asone or more small particles held in the droplet by the liquid. Forexample, when the particles 112 are composite, or multi-phase,particles, one phase of the composite may be provided in the liquid feed102 in the form of suspended precursor particles and a second phase ofthe composite may be produced in the furnace 110 from one or moreprecursors in the liquid phase of the liquid feed 102. Furthermore theprecursor particles could be included in the liquid feed 102, andtherefore also in droplets of the aerosol 108, for the purpose only ofdispersing the particles for subsequent compositional or structuralmodification during or after processing in the furnace 110.

An important aspect of the present invention is generation of theaerosol 108 with droplets of a small average size, narrow sizedistribution. In this manner, the particles 112 may be produced at adesired small size with a narrow size distribution, which areadvantageous for many applications.

The aerosol generator 106 is capable of producing the aerosol 108 suchthat it includes droplets having a weight average size in a range havinga lower limit of about 1 micron and preferably about 2 microns; and anupper limit of about 10 microns, preferably about 7 microns, morepreferably about 5 microns and most preferably about 4 microns. A weightaverage droplet size in a range of from about 2 microns to about 4microns is more preferred for most applications, with a weight averagedroplet size of about 3 microns being particularly preferred for someapplications. The aerosol generator is also capable of producing theaerosol 108 such that it includes droplets in a narrow sizedistribution. Preferably, the droplets in the aerosol are such that atleast about 70 percent (more preferably at least about 80 weight percentand most preferably at least about 85 weight percent) of the dropletsare smaller than about 10 microns and more preferably at least about 70weight percent (more preferably at least about 80 weight percent andmost preferably at least about 85 weight percent) are smaller than about5 microns. Furthermore, preferably no greater than about 30 weightpercent, more preferably no greater than about 25 weight percent andmost preferably no greater than about 20 weight percent, of the dropletsin the aerosol 108 are larger than about twice the weight averagedroplet size.

Another important aspect of the present invention is that the aerosol108 may be generated without consuming excessive amounts of the carriergas 104. The aerosol generator 106 is capable of producing the aerosol108 such that it has a high loading, or high concentration, of theliquid feed 102 in droplet form. In that regard, the aerosol 108preferably includes greater than about 1×10⁶ droplets per cubiccentimeter of the aerosol 108, more preferably greater than about 5×10⁶droplets per cubic centimeter, still more preferably greater than about1×10⁷ droplets per cubic centimeter, and most preferably greater thanabout 5×10⁷ droplets per cubic centimeter. That the aerosol generator106 can produce such a heavily loaded aerosol 108 is particularlysurprising considering the high quality of the aerosol 108 with respectto small average droplet size and narrow droplet size distribution.Typically, droplet loading in the aerosol is such that the volumetricratio of liquid feed 102 to carrier gas 104 in the aerosol 108 is largerthan about 0.04 milliliters of liquid feed 102 per liter of carrier gas104 in the aerosol 108, preferably larger than about 0.083 millilitersof liquid feed 102 per liter of carrier gas 104 in the aerosol 108, morepreferably larger than about 0.167 milliliters of liquid feed 102 perliter of carrier gas 104, still more preferably larger than about 0.25milliliters of liquid feed 102 per liter of carrier gas 104, and mostpreferably larger than about 0.333 milliliters of liquid feed 102 perliter of carrier gas 104.

This capability of the aerosol generator 106 to produce a heavily loadedaerosol 108 is even more surprising given the high droplet output rateof which the aerosol generator 106 is capable, as discussed more fullybelow. It will be appreciated that the concentration of liquid feed 102in the aerosol 108 will depend upon the specific components andattributes of the liquid feed 102 and, particularly, the size of thedroplets in the aerosol 108. For example, when the average droplet sizeis from about 2 microns to about 4 microns, the droplet loading ispreferably larger than about 0.15 milliliters of aerosol feed 102 perliter of carrier gas 104, more preferably larger than about 0.2milliliters of liquid feed 102 per liter of carrier gas 104, even morepreferably larger than about 0.2 milliliters of liquid feed 102 perliter of carrier gas 104, and most preferably larger than about 0.3milliliters of liquid feed 102 per liter of carrier gas 104. Whenreference is made herein to liters of carrier gas 104, it refers to thevolume that the carrier gas 104 would occupy under conditions ofstandard temperature and pressure.

The furnace 110 may be any suitable device for heating the aerosol 108to evaporate liquid from the droplets of the aerosol 108 and therebypermit formation of the particles 112. For most applications, maximumaverage stream temperatures in the furnace 110 will generally be in arange of from about 800° C. to about 1500° C., and preferably in therange of from about 900° C. to about 1300° C. The maximum average streamtemperature refers to the maximum average temperature that an aerosolstream attains while flowing through the furnace. This is typicallydetermined by a temperature probe inserted into the furnace. Maximumaverage stream temperatures of from about 950° C. to about 1200° C., andespecially from about 975° C. to about 1100° C., are particularlypreferred. It has been found that the use of maximum average streamtemperatures in the preferred ranges results in particularly highquality particles 112, and especially for applications where a highresistance to oxidation of palladium in the particles 112 is important.Particles 112 made with a temperature carefully controlled within thepreferred ranges show surprisingly high resistance to oxidation of thepalladium.

Although longer residence times are possible, for many applications,residence time in the heating zone of the furnace 110 of shorter thanabout 4 seconds is typical, with shorter than about 2 seconds beingpreferred, shorter than about 1 second being more preferred, shorterthan about 0.5 second being even more preferred, and shorter than about0.2 second being most preferred. The residence time should be longenough, however, to assure that the particles 112 attain the desiredmaximum average stream temperature for a given heat transfer rate. Inthat regard, with extremely short residence times, higher furnacetemperatures could be used to increase the rate of heat transfer so longas the particles 112 attain a maximum average temperature within thedesired stream temperature range. That mode of operation, however, isnot preferred. Also, it is preferred that, in most cases, the maximumaverage stream temperature not be attained in the furnace 110 untilsubstantially at the end of the heating zone in the furnace 110. Forexample, the heating zone will often include a plurality of heatingsections that are each independently controllable. The maximum averagestream temperature should typically not be attained until the finalheating section, and more preferably until substantially at the end ofthe last heating section. This is important to reduce the potential forthermophoretic losses of material. Also, it is noted that as usedherein, residence time refers to the actual time for a material to passthrough the relevant process equipment. In the case of the furnace, thisincludes the effect of increasing velocity with gas expansion due toheating.

Typically, the furnace 110 will be a tube-shaped furnace, so that theaerosol 108 moving into and through the furnace does not encounter sharpedges on which droplets could collect. Loss of droplets to collection atsharp surfaces results in a lower yield of particles 112. Moreimportant, however, the accumulation of liquid at sharp edges can resultin re-release of undesirably large droplets back into the aerosol 108,which can cause contamination of the particulate product 116 withundesirably large particles. Also, over time, such liquid collection atsharp surfaces can cause fouling of process equipment, impairing processperformance.

The furnace 110 may include a heating tube made of any suitablematerial. The tube material may be a ceramic material, for example,mullite, silica or alumina. Alternatively, the tube may be metallic.Advantages of using a metallic tube are low cost, ability to withstandsteep temperature gradients and large thermal shocks, machinability andweldability, and ease of providing a seal between the tube and otherprocess equipment. Disadvantages of using a metallic tube includelimited operating temperature and increased reactivity in some reactionsystems.

When a metallic tube is used in the furnace 110, it is preferably a highnickel content stainless steel alloy, such as a 330 stainless steel, ora nickel-based super alloy. As noted, one of the major advantages ofusing a metallic tube is that the tube is relatively easy to seal withother process equipment. In that regard, flange fittings may be weldeddirectly to the tube for connecting with other process equipment.Metallic tubes are generally preferred for making particles that do notrequire a maximum tube wall temperature of higher than about 1100° C.during particle manufacture.

When higher temperatures are required, ceramic tubes are typically used.One major problem with ceramic tubes, however, is that the tubes can bedifficult to seal with other process equipment, especially when the endsof the tubes are maintained at relatively high temperatures, as is oftenthe case with the present invention.

One configuration for sealing a ceramic tube is shown in FIGS. 2, 3 and4. The furnace 110 includes a ceramic tube 374 having an end cap 376fitted to each end of the tube 374, with a gasket 378 disposed betweencorresponding ends of the ceramic tube 374 and the end caps 376. Thegasket may be of any suitable material for sealing at the temperatureencountered at the ends of the tubes 374. Examples of gasket materialsfor sealing at temperatures below about 250° C. include silicone,TEFLON™ and VITON™. Examples of gasket materials for higher temperaturesinclude graphite, ceramic paper, thin sheet metal, and combinationsthereof.

Tension rods 380 extend over the length of the furnace 110 and throughrod holes 382 through the end caps 376. The tension rods 380 are held intension by the force of springs 384 bearing against bearing plates 386and the end caps 376. The tube 374 is, therefore, in compression due tothe force of the springs 384. The springs 384 may be compressed anydesired amount to form a seal between the end caps 376 and the ceramictube 374 through the gasket 378. As will be appreciated, by using thesprings 384, the tube 374 is free to move to some degree as it expandsupon heating and contracts upon cooling. To form the seal between theend caps 376 and the ceramic tube 374, one of the gaskets 378 is seatedin a gasket seat 388 on the side of each end cap 376 facing the tube374. A mating face 390 on the side of each of the end caps 376 facesaway from the tube 374, for mating with a flange surface for connectionwith an adjacent piece of equipment.

Also, although the present invention is described with primary referenceto a furnace reactor, which is preferred, it should be recognized that,except as noted, any other thermal reactor, including a flame reactor ora plasma reactor, could be used instead. A furnace reactor is, however,preferred, because of the generally even heating characteristic of afurnace for attaining a uniform stream temperature.

The particle collector 114, may be any suitable apparatus for collectingparticles 112 to produce the particulate product 116. One preferredembodiment of the particle collector 114 uses one or more filter toseparate the particles 112 from gas. Such a filter may be of any type,including a bag filter. Another preferred embodiment of the particlecollector uses one or more cyclone to separate the particles 112. Otherapparatus that may be used in the particle collector 114 includes anelectrostatic precipitator. Also, collection should normally occur at atemperature above the condensation temperature of the gas stream inwhich the particles 112 are suspended. Also, collection should normallybe at a temperature that is low enough to prevent significantagglomeration of the particles 112.

The process and apparatus of the present invention are well-suited forproducing commercial-size batches of extremely high qualitypalladium-containing particles. In that regard, the process and theaccompanying apparatus provide versatility for preparing powderincluding a wide variety of materials, and easily accommodate shiftingof production between different specialty batches ofpalladium-containing particles.

Of significant importance to the operation of the process of the presentinvention is the aerosol generator 106, which must be capable ofproducing a high quality aerosol with high droplet loading, aspreviously noted. With reference to FIG. 5, one embodiment of an aerosolgenerator 106 of the present invention is described. The aerosolgenerator 106 includes a plurality of ultrasonic transducer discs 120that are each mounted in a transducer housing 122. The transducerhousings 122 are mounted to a transducer mounting plate 124, creating anarray of the ultrasonic transducer discs 120. Any convenient spacing maybe used for the ultrasonic transducer discs 120. Center-to-centerspacing of the ultrasonic transducer discs 120 of about 4 centimeters isoften adequate. The aerosol generator 106, as shown in FIG. 5, includesforty-nine transducers in a 7×7 array. The array configuration is asshown in FIG. 6, which depicts the locations of the transducer housings122 mounted to the transducer mounting plate 124.

With continued reference to FIG. 5, a separator 126, in spaced relationto the transducer discs 120, is retained between a bottom retainingplate 128 and a top retaining plate 130. Gas delivery tubes 132 areconnected to gas distribution manifolds 134, which have gas deliveryports 136. The gas distribution manifolds 134 are housed within agenerator body 138 that is covered by generator lid 140. A transducerdriver 144, having circuitry for driving the transducer discs 120, iselectronically connected with the transducer discs 120 via electricalcables 146.

During operation of the aerosol generator 106, as shown in FIG. 5, thetransducer discs 120 are activated by the transducer driver 144 via theelectrical cables 146. The transducers preferably vibrate at a frequencyof from about 1 MHz to about 5 MHz, more preferably from about 1.5 MHzto about 3 MHz. Frequently used frequencies are at about 1.6 MHz andabout 2.4 MHz. Furthermore, all of the transducer discs 110 should beoperating at substantially the same frequency when an aerosol with anarrow droplet size distribution is desired. This is important becausecommercially available transducers can vary significantly in thickness,sometimes by as much as 10%. It is preferred, however, that thetransducer discs 120 operate at frequencies within a range of 5% aboveand below the median transducer frequency, more preferably within arange of 2.5%, and most preferably within a range of 1%. This can beaccomplished by careful selection of the transducer discs 120 so thatthey all preferably have thicknesses within 5% of the median transducerthickness, more preferably within 2.5%, and most preferably within 1%.

Liquid feed 102 enters through a feed inlet 148 and flows through flowchannels 150 to exit through feed outlet 152. An ultrasonicallytransmissive fluid, typically water, enters through a water inlet 154 tofill a water bath volume 156 and flow through flow channels 158 to exitthrough a water outlet 160. A proper flow rate of the ultrasonicallytransmissive fluid is necessary to cool the transducer discs 120 and toprevent overheating of the ultrasonically transmissive fluid. Ultrasonicsignals from the transducer discs 120 are transmitted, via theultrasonically transmissive fluid, across the water bath volume 156, andultimately across the separator 126, to the liquid feed 102 in flowchannels 150.

The ultrasonic signals from the ultrasonic transducer discs 120 causeatomization cones 162 to develop in the liquid feed 102 at locationscorresponding with the transducer discs 120. Carrier gas 104 isintroduced into the gas delivery tubes 132 and delivered to the vicinityof the atomization cones 162 via gas delivery ports 136. Jets of carriergas exit the gas delivery ports 136 in a direction so as to impinge onthe atomization cones 162, thereby sweeping away atomized droplets ofthe liquid feed 102 that are being generated from the atomization cones162 and creating the aerosol 108, which exits the aerosol generator 106through an aerosol exit opening 164.

Efficient use of the carrier gas 104 is an important aspect of theaerosol generator 106. The embodiment of the aerosol generator 106 shownin FIG. 5 includes two gas exit ports per atomization cone 162, with thegas ports being positioned above the liquid medium 102 over troughs thatdevelop between the atomization cones 162, such that the exiting carriergas 104 is horizontally directed at the surface of the atomization cones162, thereby efficiently distributing the carrier gas 104 to criticalportions of the liquid feed 102 for effective and efficient sweepingaway of droplets as they form about the ultrasonically energizedatomization cones 162. Furthermore, it is preferred that at least aportion of the opening of each of the gas delivery ports 136, throughwhich the carrier gas exits the gas delivery tubes, should be locatedbelow the top of the atomization cones 162 at which the carrier gas 104is directed. This relative placement of the gas delivery ports 136 isvery important to efficient use of carrier gas 104. Orientation of thegas delivery ports 136 is also important. Preferably, the gas deliveryports 136 are positioned to horizontally direct jets of the carrier gas104 at the atomization cones 162. The aerosol generator 106 permitsgeneration of the aerosol 108 with heavy loading with droplets of thecarrier liquid 102, unlike aerosol generator designs that do notefficiently focus gas delivery to the locations of droplet formation.

Another important feature of the aerosol generator 106, as shown in FIG.5, is the use of the separator 126, which protects the transducer discs120 from direct contact with the liquid feed 102, which is often highlycorrosive. The height of the separator 126 above the top of thetransducer discs 120 should normally be kept as small as possible, andis often in the range of from about 1 centimeter to about 2 centimeters.The top of the liquid feed 102 in the flow channels above the tops ofthe ultrasonic transducer discs 120 is typically in a range of fromabout 2 centimeters to about 5 centimeters, whether or not the aerosolgenerator includes the separator 126, with a distance of about 3 to 4centimeters being preferred. Although the aerosol generator 106 could bemade without the separator 126, in which case the liquid feed 102 wouldbe in direct contact with the transducer discs 120, the highly corrosivenature of the liquid feed 102 can often cause premature failure of thetransducer discs 120. The use of the separator 126, in combination withuse of the ultrasonically transmissive fluid in the water bath volume156 to provide ultrasonic coupling, significantly extending the life ofthe ultrasonic transducers 120. One disadvantage of using the separator126, however, is that the rate of droplet production from theatomization cones 162 is reduced, often by a factor of two or more,relative to designs in which the liquid feed 102 is in direct contactwith the ultrasonic transducer discs 102. Even with the separator 126,however, the aerosol generator 106 used with the present invention iscapable of producing a high quality aerosol with heavy droplet loading,as previously discussed. Suitable materials for the separator 126include, for example, polyamides (such as Kapton™ membranes from DuPont)and other polymer materials, glass, and plexiglass. The mainrequirements for the separator 126 are that it be ultrasonicallytransmissive, corrosion resistant and impermeable.

One alternative to using the separator 126 is to bind acorrosion-resistant protective coating onto the surface of theultrasonic transducer discs 120, thereby preventing the liquid feed 102from contacting the surface of the ultrasonic transducer discs 120. Whenthe ultrasonic transducer discs 120 have a protective coating, theaerosol generator 106 will typically be constructed without the waterbath volume 156 and the liquid feed 102 will flow directly over theultrasonic transducer discs 120. Examples of such protective coatingmaterials include platinum, gold, TEFLON™, epoxies and various plastics.Such coating typically significantly extends transducer life. Also, whenoperating without the separator 126, the aerosol generator 106 willtypically produce the aerosol 108 with a much higher droplet loadingthan when the separator 126 is used.

One surprising finding with operation of the aerosol generator 106 ofthe present invention is that the droplet loading in the aerosol may beaffected by the temperature of the liquid feed 102. It has been foundthat when the liquid feed 102 includes an aqueous liquid at an elevatedtemperature, the droplet loading increases significantly. Thetemperature of the liquid feed 102 is preferably higher than about 30°C., more preferably higher than about 35° C. and most preferably higherthan about 40° C. If the temperature becomes too high, however, it canhave a detrimental effect on droplet loading in the aerosol 108.Therefore, the temperature of the liquid feed 102 from which the aerosol108 is made should generally be lower than about 50° C., and preferablylower than about 45° C. The liquid feed 102 may be maintained at thedesired temperature in any suitable fashion. For example, the portion ofthe aerosol generator 106 where the liquid feed 102 is converted to theaerosol 108 could be maintained at a constant elevated temperature.Alternatively, the liquid feed 102 could be delivered to the aerosolgenerator 106 from a constant temperature bath maintained separate fromthe aerosol generator 106. When the ultrasonic generator 106 includesthe separator 126, the ultrasonically transmissive fluid adjacent theultrasonic transducer disks 120 are preferably also at an elevatedtemperature in the ranges just discussed for the liquid feed 102.

The design for the aerosol generator 106 based on an array of ultrasonictransducers is versatile and is easily modified to accommodate differentgenerator sizes for different specialty applications. The aerosolgenerator 106 may be designed to include a plurality of ultrasonictransducers in any convenient number. Even for smaller scale production,however, the aerosol generator 106 preferably has at least nineultrasonic transducers, more preferably at least 16 ultrasonictransducers, and even more preferably at least 25 ultrasonictransducers. For larger scale production, however, the aerosol generator106 includes at least 40 ultrasonic transducers, more preferably atleast 100 ultrasonic transducers, and even more preferably at least 400ultrasonic transducers. In some large volume applications, the aerosolgenerator may have at least 1000 ultrasonic transducers.

FIGS. 7-24 show component designs for an aerosol generator 106 includingan array of 400 ultrasonic transducers. Referring first to FIGS. 7 and8, the transducer mounting plate 124 is shown with a design toaccommodate an array of 400 ultrasonic transducers, arranged in foursubarrays of 100 ultrasonic transducers each. The transducer mountingplate 124 has integral vertical walls 172 for containing theultrasonically transmissive fluid, typically water, in a water bathsimilar to the water bath volume 156 described previously with referenceto FIG. 5.

As shown in FIGS. 7 and 8, four hundred transducer mounting receptacles174 are provided in the transducer mounting plate 124 for mountingultrasonic transducers for the desired array. With reference to FIG. 9,the profile of an individual transducer mounting receptacle 174 isshown. A mounting seat 176 accepts an ultrasonic transducer formounting, with a mounted ultrasonic transducer being held in place viascrew holes 178. Opposite the mounting receptacle 176 is a flaredopening 180 through which an ultrasonic signal may be transmitted forthe purpose of generating the aerosol 108, as previously described withreference to FIG. 5.

A preferred transducer mounting configuration, however, is shown in FIG.10 for another configuration for the transducer mounting plate 124. Asseen in FIG. 10, an ultrasonic transducer disc 120 is mounted to thetransducer mounting plate 124 by use of a compression screw 177 threadedinto a threaded receptacle 179. The compression screw 177 bears againstthe ultrasonic transducer disc 120, causing an o-ring 181, situated inan o-ring seat 182 on the transducer mounting plate, to be compressed toform a seal between the transducer mounting plate 124 and the ultrasonictransducer disc 120. This type of transducer mounting is particularlypreferred when the ultrasonic transducer disc 120 includes a protectivesurface coating, as discussed previously, because the seal of the o-ringto the ultrasonic transducer disc 120 will be inside of the outer edgeof the protective seal, thereby preventing liquid from penetrating underthe protective surface coating from the edges of the ultrasonictransducer disc 120.

Referring now to FIG. 11, the bottom retaining plate 128 for a 400transducer array is shown having a design for mating with the transducermounting plate 124 (shown in FIGS. 7-8). The bottom retaining plate 128has eighty openings 184, arranged in four subgroups 186 of twentyopenings 184 each. Each of the openings 184 corresponds with five of thetransducer mounting receptacles 174 (shown in FIGS. 7 and 8) when thebottom retaining plate 128 is mated with the transducer mounting plate124 to create a volume for a water bath between the transducer mountingplate 124 and the bottom retaining plate 128. The openings 184,therefore, provide a pathway for ultrasonic signals generated byultrasonic transducers to be transmitted through the bottom retainingplate.

Referring now to FIGS. 12 and 13, a liquid feed box 190 for a 400transducer array is shown having the top retaining plate 130 designed tofit over the bottom retaining plate 128 (shown in FIG. 11), with aseparator 126 (not shown) being retained between the bottom retainingplate 128 and the top retaining plate 130 when the aerosol generator 106is assembled. The liquid feed box 190 also includes vertically extendingwalls 192 for containing the liquid feed 102 when the aerosol generatoris in operation. Also shown in FIGS. 12 and 13 is the feed inlet 148 andthe feed outlet 152. An adjustable weir 198 determines the level ofliquid feed 102 in the liquid feed box 190 during operation of theaerosol generator 106.

The top retaining plate 130 of the liquid feed box 190 has eightyopenings 194 therethrough, which are arranged in four subgroups 196 oftwenty openings 194 each. The openings 194 of the top retaining plate130 correspond in size with the openings 184 of the bottom retainingplate 128 (shown in FIG. 11). When the aerosol generator 106 isassembled, the openings 194 through the top retaining plate 130 and theopenings 184 through the bottom retaining plate 128 are aligned, withthe separator 126 positioned therebetween, to permit transmission ofultrasonic signals when the aerosol generator 106 is in operation.

Referring now to FIGS. 12-14, a plurality of gas tube feed-through holes202 extend through the vertically extending walls 192 to either side ofthe assembly including the feed inlet 148 and feed outlet 152 of theliquid feed box 190. The gas tube feed-through holes 202 are designed topermit insertion therethrough of gas tubes 208 of a design as shown inFIG. 14. When the aerosol generator 106 is assembled, a gas tube 208 isinserted through each of the gas tube feed-through holes 202 so that gasdelivery ports 136 in the gas tube 208 will be properly positioned andaligned adjacent the openings 194 in the top retaining plate 130 fordelivery of gas to atomization cones that develop in the liquid feed box190 during operation of the aerosol generator 106. The gas deliveryports 136 are typically holes having a diameter of from about 1.5millimeters to about 3.5 millimeters.

Referring now to FIG. 15, a partial view of the liquid feed box 190 isshown with gas tubes 208A, 208B and 208C positioned adjacent to theopenings 194 through the top retaining plate 130. Also shown in FIG. 15are the relative locations that ultrasonic transducer discs 120 wouldoccupy when the aerosol generator 106 is assembled. As seen in FIG. 15,the gas tube 208A, which is at the edge of the array, has five gasdelivery ports 136. Each of the gas delivery ports 136 is positioned todivert carrier gas 104 to a different one of atomization cones thatdevelop over the array of ultrasonic transducer discs 120 when theaerosol generator 106 is operating. The gas tube 208B, which is one rowin from the edge of the array, is a shorter tube that has ten gasdelivery ports 136, five each on opposing sides of the gas tube 208B.The gas tube 208B, therefore, has gas delivery ports 136 for deliveringgas to atomization cones corresponding with each of ten ultrasonictransducer discs 120. The third gas tube, 208C, is a longer tube thatalso has ten gas delivery ports 136 for delivering gas to atomizationcones corresponding with ten ultrasonic transducer discs 120. The designshown in FIG. 15, therefore, includes one gas delivery port perultrasonic transducer disc 120. Although this is a lower density of gasdelivery ports 136 than for the embodiment of the aerosol generator 106shown in FIG. 5, which includes two gas delivery ports per ultrasonictransducer disc 120, the design shown in FIG. 15 is, nevertheless,capable of producing a dense, high-quality aerosol without unnecessarywaste of gas.

Referring now to FIG. 16, the flow of carrier gas 104 relative toatomization cones 162 during operation of the aerosol generator 106having a gas distribution configuration to deliver carrier gas 104 fromgas delivery ports on both sides of the gas tubes 208, as was shown forthe gas tubes 208A, 208B and 208C in the gas distribution configurationshown in FIG. 14. The carrier gas 104 sweeps both directions from eachof the gas tubes 208.

An alternative, and preferred, flow for carrier gas 104 is shown in FIG.17. As shown in FIG. 17, carrier gas 104 is delivered from only one sideof each of the gas tubes 208. This results in a sweep of carrier gasfrom all of the gas tubes 208 toward a central area 212. This results ina more uniform flow pattern for aerosol generation that maysignificantly enhance the efficiency with which the carrier gas 104 isused to produce an aerosol. The aerosol that is generated, therefore,tends to be more heavily loaded with liquid droplets.

Another configuration for distributing carrier gas in the aerosolgenerator 106 is shown in FIGS. 18 and 19. In this configuration, thegas tubes 208 are hung from a gas distribution plate 216 adjacent gasflow holes 218 through the gas distribution plate 216. In the aerosolgenerator 106, the gas distribution plate 216 would be mounted above theliquid feed, with the gas flow holes positioned to each correspond withan underlying ultrasonic transducer. Referring specifically to FIG. 19,when the ultrasonic generator 106 is in operation, atomization cones 162develop through the gas flow holes 218, and the gas tubes 208 arelocated such that carrier gas 104 exiting from ports in the gas tubes208 impinge on the atomization cones and flow upward through the gasflow holes. The gas flow holes 218, therefore, act to assist inefficiently distributing the carrier gas 104 about the atomization cones162 for aerosol formation. It should be appreciated that the gasdistribution plates 218 can be made to accommodate any number of the gastubes 208 and gas flow holes 218. For convenience of illustration, theembodiment shown in FIGS. 18 and 19 shows a design having only two ofthe gas tubes 208 and only 16 of the gas flow holes 218. Also, it shouldbe appreciated that the gas distribution plate 216 could be used alone,without the gas tubes 208. In that case, a slight positive pressure ofcarrier gas 104 would be maintained under the gas distribution plate 216and the gas flow holes 218 would be sized to maintain the propervelocity of carrier gas 104 through the gas flow holes 218 for efficientaerosol generation. Because of the relative complexity of operating inthat mode, however, it is not preferred.

Aerosol generation may also be enhanced through mounting of ultrasonictransducers at a slight angle and directing the carrier gas at resultingatomization cones such that the atomization cones are tilting in thesame direction as the direction of flow of carrier gas. Referring toFIG. 20, an ultrasonic transducer disc 120 is shown. The ultrasonictransducer disc 120 is tilted at a tilt angle 114 (typically less than10 degrees), so that the atomization cone 162 will also have a tilt. Itis preferred that the direction of flow of the carrier gas 104 directedat the atomization cone 162 is in the same direction as the tilt of theatomization cone 162.

Referring now to FIGS. 21 and 22, a gas manifold 220 is shown fordistributing gas to the gas tubes 208 in a 400 transducer array design.The gas manifold 220 includes a gas distribution box 222 and pipingstubs 224 for connection with gas tubes 208 (shown in FIG. 14). Insidethe gas distribution box 222 are two gas distribution plates 226 thatform a flow path to assist in distributing the gas equally throughoutthe gas distribution box 222, to promote substantially equal delivery ofgas through the piping stubs 224. The gas manifold 220, as shown inFIGS. 21 and 22, is designed to feed eleven gas tubes 208. For the 400transducer design, a total of four gas manifolds 220 are required.

Referring now to FIGS. 23 and 24, the generator lid 140 is shown for a400 transducer array design. The generator lid 140 mates with and coversthe liquid feed box 190 (shown in FIGS. 12 and 13). The generator lid140, as shown in FIGS. 23 and 24, has a hood design to permit easycollection of the aerosol 108 without subjecting droplets in the aerosol108 to sharp edges on which droplets may coalesce and be lost, andpossibly interfere with the proper operation of the aerosol generator106. When the aerosol generator 106 is in operation, the aerosol 108would be withdrawn via the aerosol exit opening 164 through thegenerator cover 140.

Although the aerosol generator 106 produces a high quality aerosol 108having a high droplet loading, it is often desirable to furtherconcentrate the aerosol 108 prior to introduction into the furnace 110.Referring now to FIG. 25, a process flow diagram is shown for oneembodiment of the present invention involving such concentration of theaerosol 108. As shown in FIG. 25, the aerosol 108 from the aerosolgenerator 106 is sent to an aerosol concentrator 236 where excesscarrier gas 238 is withdrawn from the aerosol 108 to produce aconcentrated aerosol 240, which is then fed to the furnace 110.

The aerosol concentrator 236 typically includes one or more virtualimpactors capable of concentrating droplets in the aerosol 108 by afactor of greater than about 2, preferably by a factor of greater thanabout 5, and more preferably by a factor of greater than about 10, toproduce the concentrated aerosol 240. According to the presentinvention, the concentrated aerosol 240 should typically contain greaterthan about 1×10⁷ droplets per cubic centimeter, and more preferably fromabout 5×10⁷ to about 5×10⁸ droplets per cubic centimeter. Aconcentration of about 1×10⁸ droplets per cubic centimeter of theconcentrated aerosol is particularly preferred, because when theconcentrated aerosol 240 is loaded more heavily than that, then thefrequency of collisions between droplets becomes large enough to impairthe properties of the concentrated aerosol 240, resulting in potentialcontamination of the particulate product 116 with an undesirably largequantity of over-sized particles. For example, if the aerosol 108 has aconcentration of about 1×10⁷ droplets per cubic centimeter, and theaerosol concentrator 236 concentrates droplets by a factor of 10, thenthe concentrated aerosol 240 will have a concentration of about 1×10⁸droplets per cubic centimeter. Stated another way, for example, when theaerosol generator generates the aerosol 108 with a droplet loading ofabout 0.167 milliliters liquid feed 102 per liter of carrier gas 104,the concentrated aerosol 240 would be loaded with about 1.67 millilitersof liquid feed 102 per liter of carrier gas 104, assuming the aerosol108 is concentrated by a factor of 10.

Having a high droplet loading in aerosol feed to the furnace providesthe important advantage of reducing the heating demand on the furnace110 and the size of flow conduits required through the furnace. Also,other advantages of having a dense aerosol include a reduction in thedemands on cooling and particle collection components, permittingsignificant equipment and operational savings. Furthermore, as systemcomponents are reduced in size, powder holdup within the system isreduced, which is also desirable. Concentration of the aerosol streamprior to entry into the furnace 110, therefore, provides a substantialadvantage relative to processes that utilize less concentrated aerosolstreams.

The excess carrier gas 238 that is removed in the aerosol concentrator236 typically includes extremely small droplets that are also removedfrom the aerosol 108. Preferably, the droplets removed with the excesscarrier gas 238 have a weight average size of smaller than about 1.5microns, and more preferably smaller than about 1 micron and thedroplets retained in the concentrated aerosol 240 have an averagedroplet size of larger than about 2 microns. For example, a virtualimpactor sized to treat an aerosol stream having a weight averagedroplet size of about three microns might be designed to remove with theexcess carrier gas 238 most droplets smaller than about 1.5 microns insize. Other designs are also possible. When using the aerosol generator106 with the present invention, however, the loss of these very smalldroplets in the aerosol concentrator 236 will typically constitute nomore than about 10 percent by weight, and more preferably no more thanabout 5 percent by weight, of the droplets originally in the aerosolstream that is fed to the concentrator 236. Although the aerosolconcentrator 236 is useful in some situations, it is normally notrequired with the process of the present invention, because the aerosolgenerator 106 is capable, in most circumstances, of generating anaerosol stream that is sufficiently dense. So long as the aerosol streamcoming out of the aerosol generator 102 is sufficiently dense, it ispreferred that the aerosol concentrator not be used. It is a significantadvantage of the present invention that the aerosol generator 106normally generates such a dense aerosol stream that the aerosolconcentrator 236 is not needed. Therefore, the complexity of operationof the aerosol concentrator 236 and accompanying liquid losses maytypically be avoided.

It is important that the aerosol stream (whether it has beenconcentrated or not) that is fed to the furnace 110 have a high dropletflow rate and high droplet loading as would be required for mostindustrial applications. With the present invention, the aerosol streamfed to the furnace preferably includes a droplet flow of greater thanabout 0.5 liters per hour, more preferably greater than about 2 litersper hour, still more preferably greater than about 5 liters per hour,even more preferably greater than about 10 liters per hour, particularlygreater than about 50 liters per hour and most preferably greater thanabout 100 liters per hour; and with the droplet loading being typicallygreater than about 0.04 milliliters of droplets per liter of carriergas, preferably greater than about 0.083 milliliters of droplets perliter of carrier gas 104, more preferably greater than about 0.167milliliters of droplets per liter of carrier gas 104, still morepreferably greater than about 0.25 milliliters of droplets per liter ofcarrier gas 104, particularly greater than about 0.33 milliliters ofdroplets per liter of carrier gas 104 and most preferably greater thanabout 0.83 milliliters of droplets per liter of carrier gas 104.

One embodiment of a virtual impactor that could be used as the aerosolconcentrator 236 will now be described with reference to FIGS. 26-32. Avirtual impactor 246 includes an upstream plate assembly 248 (detailsshown in FIGS. 27-29) and a downstream plate assembly 250 (details shownin FIGS. 25-32), with a concentrating chamber 262 located between theupstream plate assembly 248 and the downstream plate assembly 250.

Through the upstream plate assembly 248 are a plurality of verticallyextending inlet slits 254. The downstream plate assembly 250 includes aplurality of vertically extending exit slits 256 that are in alignmentwith the inlet slits 254. The exit slits 256 are, however, slightlywider than the inlet slits 254. The downstream plate assembly 250 alsoincludes flow channels 258 that extend substantially across the width ofthe entire downstream plate assembly 250, with each flow channel 258being adjacent to an excess gas withdrawal port 260.

During operation, the aerosol 108 passes through the inlet slits 254 andinto the concentrating chamber 262. Excess carrier gas 238 is withdrawnfrom the concentrating chamber 262 via the excess gas withdrawal ports260. The withdrawn excess carrier gas 238 then exits via a gas duct port264. That portion of the aerosol 108 that is not withdrawn through theexcess gas withdrawal ports 260 passes through the exit slits 256 andthe flow channels 258 to form the concentrated aerosol 240. Thosedroplets passing across the concentrating chamber 262 and through theexit slits 256 are those droplets of a large enough size to havesufficient momentum to resist being withdrawn with the excess carriergas 238.

As seen best in FIGS. 27-32, the inlet slits 254 of the upstream plateassembly 248 include inlet nozzle extension portions 266 that extendoutward from the plate surface 268 of the upstream plate assembly 248.The exit slits 256 of the downstream plate assembly 250 include exitnozzle extension portions 270 extending outward from a plate surface 272of the downstream plate assembly 250. These nozzle extension portions266 and 270 are important for operation of the virtual impactor 246,because having these nozzle extension portions 266 and 270 permits avery close spacing to be attained between the inlet slits 254 and theexit slits 256 across the concentrating chamber 262, while alsoproviding a relatively large space in the concentrating chamber 262 tofacilitate efficient removal of the excess carrier gas 238.

Also as best seen in FIGS. 27-32, the inlet slits 254 have widths thatflare outward toward the side of the upstream plate assembly 248 that isfirst encountered by the aerosol 108 during operation. This flaredconfiguration reduces the sharpness of surfaces encountered by theaerosol 108, reducing the loss of aerosol droplets and potentialinterference from liquid buildup that could occur if sharp surfaces werepresent. Likewise, the exit slits 256 have a width that flares outwardtowards the flow channels 258, thereby allowing the concentrated aerosol240 to expand into the flow channels 258 without encountering sharpedges that could cause problems.

As noted previously, both the inlet slits 254 of the upstream plateassembly 248 and the exit slits 256 of the downstream plate assembly 250are vertically extending. This configuration is advantageous forpermitting liquid that may collect around the inlet slits 254 and theexit slits 256 to drain away. The inlet slits 254 and the exit slits 256need not, however, have a perfectly vertical orientation. Rather, it isoften desirable to slant the slits backward (sloping upward and away inthe direction of flow) by about five to ten degrees relative tovertical, to enhance draining of liquid off of the upstream plateassembly 248 and the downstream plate assembly 250. This drainagefunction of the vertically extending configuration of the inlet slits254 and the outlet slits 256 also inhibits liquid build-up in thevicinity of the inlet slits 248 and the exit slits 250, which liquidbuild-up could result in the release of undesirably large droplets intothe concentrated aerosol 240.

As discussed previously, the aerosol generator 106 of the presentinvention produces a concentrated, high quality aerosol of micro-sizeddroplets having a relatively narrow size distribution. It has beenfound, however, that for many applications the process of the presentinvention is significantly enhanced by further classifying by size thedroplets in the aerosol 108 prior to introduction of the droplets intothe furnace 110. In this manner, the size and size distribution ofparticles in the particulate product 116 are further controlled.

Referring now to FIG. 33, a process flow diagram is shown for oneembodiment of the process of the present invention including suchdroplet classification. As shown in FIG. 33, the aerosol 108 from theaerosol generator 106 goes to a droplet classifier 280 where oversizeddroplets are removed from the aerosol 108 to prepare a classifiedaerosol 282. Liquid 284 from the oversized droplets that are beingremoved is drained from the droplet classifier 280. This drained liquid284 may advantageously be recycled for use in preparing additionalliquid feed 102.

Any suitable droplet classifier may be used for removing droplets abovea predetermined size. For example, a cyclone could be used to removeover-size droplets. A preferred droplet classifier for manyapplications, however, is an impactor. One embodiment of an impactor foruse with the present invention will now be described with reference toFIGS. 34-38.

As seen in FIG. 34, an impactor 288 has disposed in a flow conduit 286 aflow control plate 290 and an impactor plate assembly 292. The flowcontrol plate 290 is conveniently mounted on a mounting plate 294.

The flow control plate 290 is used to channel the flow of the aerosolstream toward the impactor plate assembly 292 in a manner withcontrolled flow characteristics that are desirable for proper impactionof oversize droplets on the impactor plate assembly 292 for removalthrough the drains 296 and 314. One embodiment of the flow control plate290 is shown in FIG. 35. The flow control plate 290 has an array ofcircular flow ports 296 for channeling flow of the aerosol 108 towardsthe impactor plate assembly 292 with the desired flow characteristics.

Details of the mounting plate 294 are shown in FIG. 36. The mountingplate 294 has a mounting flange 298 with a large diameter flow opening300 passing therethrough to permit access of the aerosol 108 to the flowports 296 of the flow control plate 290 (shown in FIG. 35).

Referring now to FIGS. 37 and 38, one embodiment of an impactor plateassembly 292 is shown. The impactor plate assembly 292 includes animpactor plate 302 and mounting brackets 304 and 306 used to mount theimpactor plate 302 inside of the flow conduit 286. The impactor plate302 and the flow channel plate 290 are designed so that droplets largerthan a predetermined size will have momentum that is too large for thoseparticles to change flow direction to navigate around the impactor plate302.

During operation of the impactor 288, the aerosol 108 from the aerosolgenerator 106 passes through the upstream flow control plate 290. Mostof the droplets in the aerosol navigate around the impactor plate 302and exit the impactor 288 through the downstream flow control plate 290in the classified aerosol 282. Droplets in the aerosol 108 that are toolarge to navigate around the impactor plate 302 will impact on theimpactor plate 302 and drain through the drain 296 to be collected withthe drained liquid 284 (as shown in FIG. 34).

The configuration of the impactor plate 302 shown in FIG. 33 representsonly one of many possible configurations for the impactor plate 302. Forexample, the impactor 288 could include an upstream flow control plate290 having vertically extending flow slits therethrough that are offsetfrom vertically extending flow slits through the impactor plate 302,such that droplets too large to navigate the change in flow due to theoffset of the flow slits between the flow control plate 290 and theimpactor plate 302 would impact on the impactor plate 302 to be drainedaway. Other designs are also possible.

In a preferred embodiment of the present invention, the dropletclassifier 280 is typically designed to remove droplets from the aerosol108 that are larger than about 15 microns in size, more preferably toremove droplets larger than about 10 microns in size, even morepreferably to remove droplets of a size larger than about 8 microns insize and most preferably to remove droplets larger than about 5 micronsin size. The droplet classification size in the droplet classifier ispreferably smaller than about 15 microns, more preferably smaller thanabout 10 microns, even more preferably smaller than about 8 microns andmost preferably smaller than about 5 microns. The classification size,also called the classification cut point, is that size at which half ofthe droplets of that size are removed and half of the droplets of thatsize are retained. Depending upon the specific application, however, thedroplet classification size may be varied, such as by changing thespacing between the impactor plate 302 and the flow control plate 290 orincreasing or decreasing aerosol velocity through the jets in the flowcontrol plate 290. Because the aerosol generator 106 of the presentinvention initially produces a high quality aerosol 108, having arelatively narrow size distribution of droplets, typically less thanabout 30 weight percent of liquid feed 102 in the aerosol 108 is removedas the drain liquid 284 in the droplet classifier 288, with preferablyless than about 25 weight percent being removed, even more preferablyless than about 20 weight percent being removed and most preferably lessthan about 15 weight percent being removed. Minimizing the removal ofliquid feed 102 from the aerosol 108 is particularly important forcommercial applications to increase the yield of high qualityparticulate product 116. It should be noted, however, that because ofthe superior performance of the aerosol generator 106, it is frequentlynot required to use an impactor or other droplet classifier to obtain adesired absence of oversize droplets to the furnace. This is a majoradvantage, because the added complexity and liquid losses accompanyinguse of an impactor may often be avoided with the process of the presentinvention.

Sometimes it is desirable to use both the aerosol concentrator 236 andthe droplet classifier 280 to produce an extremely high quality aerosolstream for introduction into the furnace for the production of particlesof highly controlled size and size distribution. Referring now to FIG.39, one embodiment of the present invention is shown incorporating boththe virtual impactor 246 and the impactor 288. Basic components of thevirtual impactor 246 and the impactor 288, as shown in FIG. 39, aresubstantially as previously described with reference to FIGS. 26-38. Asseen in FIG. 39, the aerosol 108 from the aerosol generator 106 is fedto the virtual impactor 246 where the aerosol stream is concentrated toproduce the concentrated aerosol 240. The concentrated aerosol 240 isthen fed to the impactor 288 to remove large droplets therefrom andproduce the classified aerosol 282, which may then be fed to the furnace110. Also, it should be noted that by using both a virtual impactor andan impactor, both undesirably large and undesirably small droplets areremoved, thereby producing a classified aerosol with a very narrowdroplet size distribution. Also, the order of the aerosol concentratorand the aerosol classifier could be reversed, so that the aerosolconcentrator 236 follows the aerosol classifier 280.

One important feature of the design shown in FIG. 39 is theincorporation of drains 310, 312, 314, 316 and 296 at strategiclocations. These drains are extremely important for industrial-scaleparticle production because buildup of liquid in the process equipmentcan significantly impair the quality of the particulate product 116 thatis produced. In that regard, drain 310 drains liquid away from the inletside of the first plate assembly 248 of the virtual impactor 246. Drain312 drains liquid away from the inside of the concentrating chamber 262in the virtual impactor 246 and drain 314 removes liquid that depositsout of the excess carrier gas 238. Drain 316 removes liquid from thevicinity of the inlet side of the flow control plate 290 of theimpactor, while the drain 296 removes liquid from the vicinity of theimpactor plate 302. Without these drains 310, 312, 314, 316 and 296, theperformance of the apparatus shown in FIG. 39 would be significantlyimpaired. All liquids drained in the drains 310, 312, 314, 316 and 296may advantageously be recycled for use to prepare the liquid feed 102.

With some applications of the process of the present invention, it maybe possible to collect the particles 112 directly from the output of thefurnace 110. More often, however, it will be desirable to cool theparticles 112 exiting the furnace 110 prior to collection of theparticles 112 in the particle collector 114. Referring now to FIG. 40,one embodiment of the process of the present invention is shown in whichthe particles 112 exiting the furnace 110 are sent to a particle cooler320 to produce a cooled particle stream 322, which is then feed to theparticle collector 114. Although the particle cooler 320 may be anycooling apparatus capable of cooling the particles 112 to the desiredtemperature for introduction into the particle collector 114,traditional heat exchanger designs are not preferred. This is because atraditional heat exchanger design ordinarily directly subjects theaerosol stream, in which the hot particles 112 are suspended, to coolsurfaces. In that situation, significant losses of the particles 112occur due to thermophoretic deposition of the hot particles 112 on thecool surfaces of the heat exchanger. According to the present invention,a gas quench apparatus is provided for use as the particle cooler 320that significantly reduces thermophoretic losses compared to atraditional heat exchanger.

Referring now to FIGS. 41-43, one embodiment of a gas quench cooler 330is shown. The gas quench cooler includes a perforated conduit 332 housedinside of a cooler housing 334 with an annular space 336 located betweenthe cooler housing 334 and the perforated conduit 332. In fluidcommunication with the annular space 336 is a quench gas inlet box 338,inside of which is disposed a portion of an aerosol outlet conduit 340.The perforated conduit 332 extends between the aerosol outlet conduit340 and an aerosol inlet conduit 342. Attached to an opening into thequench gas inlet box 338 are two quench gas feed tubes 344. Referringspecifically to FIG. 43, the perforated tube 332 is shown. Theperforated tube 332 has a plurality of openings 345. The openings 345,when the perforated conduit 332 is assembled into the gas quench cooler330, permit the flow of quench gas 346 from the annular space 336 intothe interior space 348 of the perforated conduit 332. Although theopenings 345 are shown as being round holes, any shape of opening couldbe used, such as slits. Also, the perforated conduit 332 could be aporous screen. Two heat radiation shields 347 prevent downstream radiantheating from the furnace. In most instances, however, it will not benecessary to include the heat radiation shields 347, because downstreamradiant heating from the furnace is normally not a significant problem.Use of the heat radiation shields 347 is not preferred due toparticulate losses that accompany their use.

With continued reference to FIGS. 41-43, operation of the gas quenchcooler 330 will now be described. During operation, the particles 112,carried by and dispersed in a gas stream, enter the gas quench cooler330 through the aerosol inlet conduit 342 and flow into the interiorspace 348 of perforated conduit 332. Quench gas 346 is introducedthrough the quench gas feed tubes 344 into the quench gas inlet box 338.Quench gas 346 entering the quench gas inlet box 338 encounters theouter surface of the aerosol outlet conduit 340, forcing the quench gas346 to flow, in a spiraling, swirling manner, into the annular space336, where the quench gas 346 flows through the openings 345 through thewalls of the perforated conduit 332. Preferably, the gas 346 retainssome swirling motion even after passing into the interior space 348. Inthis way, the particles 112 are quickly cooled with low losses ofparticles to the walls of the gas quench cooler 330. In this manner, thequench gas 346 enters in a radial direction into the interior space 348of the perforated conduit 332 around the entire periphery, orcircumference, of the perforated conduit 332 and over the entire lengthof the perforated conduit 332. The cool quench gas 346 mixes with andcools the hot particles 112, which then exit through the aerosol outletconduit 340 as the cooled particle stream 322. The cooled particlestream 322 can then be sent to the particle collector 114 for particlecollection. The temperature of the cooled particle stream 322 iscontrolled by introducing more or less quench gas. Also, as shown inFIG. 41, the quench gas 346 is fed into the quench cooler 330 in counterflow to flow of the particles. Alternatively, the quench cooler could bedesigned so that the quench gas 346 is fed into the quench cooler inconcurrent flow with the flow of the particles 112. The amount of quenchgas 346 fed to the gas quench cooler 330 will depend upon the specificmaterial being made and the specific operating conditions. The quantityof quench gas 346 used, however, must be sufficient to reduce thetemperature of the aerosol steam including the particles 112 to thedesired temperature. Typically, the particles 112 are cooled to atemperature at least below about 200° C., and often lower. The onlylimitation on how much the particles 112 are cooled is that the cooledparticle stream 322 must be at a temperature that is above thecondensation temperature for water as another condensible vapor in thestream. The temperature of the cooled particle stream 322 is often at atemperature of from about 50° C. to about 120° C.

Because of the entry of quench gas 346 into the interior space 348 ofthe perforated conduit 322 in a radial direction about the entirecircumference and length of the perforated conduit 322, a buffer of thecool quench gas 346 is formed about the inner wall of the perforatedconduit 332, thereby significantly inhibiting the loss of hot particles112 due to thermophoretic deposition on the cool wall of the perforatedconduit 332. In operation, the quench gas 346 exiting the openings 345and entering into the interior space 348 should have a radial velocity(velocity inward toward the center of the circular cross-section of theperforated conduit 332) of larger than the thermophoretic velocity ofthe particles 112 inside the perforated conduit 332 in a directionradially outward toward the perforated wall of the perforated conduit332.

As seen in FIGS. 41-43, the gas quench cooler 330 includes a flow pathfor the particles 112 through the gas quench cooler of a substantiallyconstant cross-sectional shape and area. Preferably, the flow paththrough the gas quench cooler 330 will have the same cross-sectionalshape and area as the flow path through the furnace 110 and through theconduit delivering the aerosol 108 from the aerosol generator 106 to thefurnace 110. In one embodiment, however, it may be necessary to reducethe cross-sectional area available for flow prior to the particlecollector 114. This is the case, for example, when the particlecollector includes a cyclone for separating particles in the cooledparticle stream 322 from gas in the cooled particle stream 322. This isbecause of the high inlet velocity requirements into cyclone separators.

Referring now to FIG. 44, one embodiment of the gas quench cooler 330 isshown in combination with a cyclone separator 392. The perforatedconduit 332 has a continuously decreasing cross-sectional area for flowto increase the velocity of flow to the proper value for the feed tocyclone separator 392. Attached to the cyclone separator 392 is a bagfilter 394 for final clean-up of overflow from the cyclone separator392. Separated particles exit with underflow from the cyclone separator392 and may be collected in any convenient container. The use of cycloneseparation is particularly preferred for powder having a weight averagesize of larger than about 1 micron, although a series of cyclones maysometimes be needed to get the desired degree of separation. Cycloneseparation is particularly preferred for powders having a weight averagesize of larger than about 1.5 microns. Also, cyclone separation is bestsuited for high density materials. Preferably, when particles areseparated using a cyclone, the particles are of a composition withspecific gravity of greater than about 5.

In an additional embodiment, the process of the present invention canalso incorporate compositional modification of the particles 112 exitingthe furnace. Most commonly, the compositional modification will involveforming on the particles 112 a material phase that is different thanthat of the particles 112, such as by coating the particles 112 with acoating material. One embodiment of the process of the present inventionincorporating particle coating is shown in FIG. 45. As shown in FIG. 45,the particles 112 exiting from the furnace 110 go to a particle coater350 where a coating is placed over the outer surface of the particles112 to form coated particles 352, which are then sent to the particlecollector 114 for preparation of the particulate product 116.

In the particle coater 350, the particles 112 are coated using anysuitable particle coating technology, such as by gas-to-particleconversion. Preferably, however, the coating is accomplished by chemicalvapor deposition (CVD) and/or physical vapor deposition (PVD). In CVDcoating, one or more vapor phase coating precursors are reacted to forma surface coating on the particles 112. Preferred coatings deposited byCVD include oxides, such as silica, and elemental metals. In PVDcoating, coating material physically deposits on the surface of theparticles 112. Preferred coatings deposited by PVD include organicmaterials and elemental metals, such as elemental silver, copper andgold. Another possible surface coating method is surface conversion ofthe surface portion of the particles 112 by reaction with a vapor phasereactant to convert a surface portion of the particles to a differentmaterial than that originally contained in the particles 112. Althoughany suitable apparatus may be used for the particle coater 350, when agaseous coating feed involving coating precursors is used, such as forCVD and PVD, feed of the gaseous coating feed is introduced through acircumferentially perforated conduit, such as was described for thequench cooler 330 with reference to FIGS. 41-44. In some instances, thequench cooler 330 may also act as the particle coater 350, when coatingmaterial precursors are included in the quench gas 346.

With continued reference primarily to FIG. 45, in a preferredembodiment, when the particles 112 are coated according to the processof the present invention, the particles 112 are also manufactured viathe aerosol process of the present invention, as previously described.The process of the present invention can, however, be used to coatparticles that have been premanufactured by a different process, such asby a liquid precipitation route. When coating particles that have beenpremanufactured by a different route, such as by liquid precipitation,it is preferred that the particles remain in a dispersed state from thetime of manufacture to the time that the particles are introduced inslurry form into the aerosol generator 106 for preparation of theaerosol 108 to form the dry particles 112 in the furnace 110, whichparticles 112 can then be coated in the particle coater 350. Maintainingparticles in a dispersed state from manufacture through coating avoidsproblems associated with agglomeration and redispersion of particles ifparticles must be redispersed in the liquid feed 102 for feed to theaerosol generator 106. For example, for particles originallyprecipitated from a liquid medium, the liquid medium containing thesuspended precipitated particles could be used to form the liquid feed102 to the aerosol generator 106. It should be noted that the particlecoater 350 could be an integral extension of the furnace 110 or could bea separate piece of equipment.

In a further embodiment of the present invention, following preparationof the particles 112 in the furnace 110, the particles 112 may then bestructurally modified to impart desired physical properties prior toparticle collection. Referring now to FIG. 46, one embodiment of theprocess of the present invention is shown including such structuralparticle modification. The particles 112 exiting the furnace 110 go to aparticle modifier 360 where the particles are structurally modified toform modified particles 362, which are then sent to the particlecollector 114 for preparation of the particulate product 116. Theparticle modifier 360 is typically a furnace, such as an annealingfurnace, which may be integral with the furnace 110 or may be a separateheating device. Regardless, it is important that the particle modifier360 have temperature control that is independent of the furnace 110, sothat the proper conditions for particle modification may be providedseparate from conditions required of the furnace 110 to prepare theparticles 112. The particle modifier 360, therefore, typically providesa temperature controlled environment and necessary residence time toeffect the desired structural modification of the particles 112.

The structural modification that occurs in the particle modifier 360 maybe any modification to the crystalline structure or morphology of theparticles 112. For example, the particles 112 may be annealed in theparticle modifier 360 to densify the particles 112 or to recrystallizethe particles 112 into a polycrystalline or single crystalline form.Also, especially in the case of composite particles 112, the particlesmay be annealed for a sufficient time to permit redistribution withinthe particles 112 of different material phases.

The initial morphology of composite particles made in the furnace 110,according to the present invention, could take a variety of forms,depending upon the specified materials involved and the specificprocessing conditions. Examples of some possible composite particlemorphologies, manufacturable according to the present invention areshown in FIG. 47. These morphologies could be of the particles asinitially produced in the furnace 110 or that result from structuralmodification in the particle modifier 360. Furthermore, the compositeparticles could include a mixture of the morphological attributes shownin FIG. 47.

Referring now to FIG. 48, an embodiment of the apparatus of the presentinvention is shown that includes the aerosol generator 106 (in the formof the 400 transducer array design), the aerosol concentrator 236 (inthe form of a virtual impactor), the droplet classifier 280 (in the formof an impactor), the furnace 110, the particle cooler 320 (in the formof a gas quench cooler) and the particle collector 114 (in the form of abag filter). All process equipment components are connected viaappropriate flow conduits that are substantially free of sharp edgesthat could detrimentally cause liquid accumulations in the apparatus.Also, it should be noted that flex connectors 370 are used upstream anddownstream of the aerosol concentrator 236 and the droplet classifier280. By using the flex connectors 370, it is possible to vary the angleof slant of vertically extending slits in the aerosol concentrator 236and/or the droplet classifier 280. In this way, a desired slant for thevertically extending slits may be set to optimize the drainingcharacteristics off the vertically extending slits.

Aerosol generation with the process of the present invention has thusfar been described with respect to the ultrasonic aerosol generator. Useof the ultrasonic generator is preferred for the process of the presentinvention because of the extremely high quality and dense aerosolgenerated. In some instances, however, the aerosol generator for theprocess of the present invention may have a different design dependingupon the specific application. For example, when larger particles aredesired, such as those having a weight average size of larger than about3 microns, a spray nozzle atomizer may be preferred. Forsmaller-particle applications, however, and particularly for thoseapplications to produce particles smaller than about 3 microns, andpreferably smaller than about 2 microns in size, as is generally desiredwith the particles of the present invention, the ultrasonic generator,as described herein, is particularly preferred. In that regard, theultrasonic generator of the present invention is particularly preferredfor when making particles with a weight average size of from about 0.2micron to about 3 microns.

Although ultrasonic aerosol generators have been used for medicalapplications and home humidifiers, use of ultrasonic generators forspray pyrolysis particle manufacture has largely been confined tosmall-scale, experimental situations. The ultrasonic aerosol generatorof the present invention described with reference to FIGS. 5-24,however, is well suited for commercial production of high qualitypowders with a small average size and a narrow size distribution. Inthat regard, the aerosol generator produces a high quality aerosol, withheavy droplet loading and at a high rate of production. Such acombination of small droplet size, narrow size distribution, heavydroplet loading, and high production rate provide significant advantagesover existing aerosol generators that usually suffer from at least oneof inadequately narrow size distribution, undesirably low dropletloading, or unacceptably low production rate.

Through the careful and controlled design of the ultrasonic generator ofthe present invention, an aerosol may be produced typically havinggreater than about 70 weight percent (and preferably greater than about80 weight percent) of droplets in the size range of from about 1 micronto about 10 microns, preferably in a size range of from about 1 micronto about 5 microns and more preferably from about 2 microns to about 4microns. Also, the ultrasonic generator of the present invention iscapable of delivering high output rates of liquid feed in the aerosol.The rate of liquid feed, at the high liquid loadings previouslydescribed, is preferably greater than about 25 milliliters per hour pertransducer, more preferably greater than about 37.5 milliliters per hourper transducer, even more preferably greater than about 50 millilitersper hour per transducer and most preferably greater than about 100millimeters per hour per transducer. This high level of performance isdesirable for commercial operations and is accomplished with the presentinvention with a relatively simple design including a single precursorbath over an array of ultrasonic transducers. The ultrasonic generatoris made for high aerosol production rates at a high droplet loading, andwith a narrow size distribution of droplets. The generator preferablyproduces an aerosol at a rate of greater than about 0.5 liter per hourof droplets, more preferably greater than about 2 liters per hour ofdroplets, still more preferably greater than about 5 liters per hour ofdroplets, even more preferably greater than about 10 liters per hour ofdroplets and most preferably greater than about 40 liters per hour ofdroplets. For example, when the aerosol generator has a 400 transducerdesign, as described with reference to FIGS. 7-24, the aerosol generatoris capable of producing a high quality aerosol having high dropletloading as previously described, at a total production rate ofpreferably greater than about 10 liters per hour of liquid feed, morepreferably greater than about 15 liters per hour of liquid feed, evenmore preferably greater than about 20 liters per hour of liquid feed andmost preferably greater than about 40 liters per hour of liquid feed.

Under most operating conditions, when using such an aerosol generator,total particulate product produced is preferably greater than about 0.5gram per hour per transducer, more preferably greater than about 0.75gram per hour per transducer, even more preferably greater than about1.0 gram per hour per transducer and most preferably greater than about2.0 grams per hour per transducer.

The concentrations of soluble precursors in the liquid feed 102 willvary depending upon the particular materials involved and the particularparticle composition and particle morphology desired. For mostapplications, when soluble precursor(s) are used, the solubleprecursor(s) are present at a concentration of from about 1-50 weightpercent of the liquid feed. 102. In any event, however, when solubleprecursors are used, the precursors should be at a low enoughconcentration to permit the liquid feed to be ultrasonically atomizedand to prevent premature precipitation of materials from the liquid feed102. The concentration of suspended particulate precursors will alsovary depending upon the particular materials involved in the particularapplication.

One significant aspect of the process of the present invention formanufacturing particulate materials is the unique flow characteristicsencountered in the furnace relative to laboratory scale systems. Themaximum Reynolds number attained for flow in the furnace 110 with thepresent invention is very high, typically in excess of 500, preferablyin excess of 1,000 and more preferably in excess of 2,000. In mostinstances, however, the maximum Reynolds number for flow in the furnacewill not exceed 10,000, and preferably will not exceed 5,000. This issignificantly different from lab-scale systems where the Reynolds numberfor flow in a reactor is typically lower than 50 and rarely exceeds 100.

The Reynolds number is a dimensionless quantity characterizing flow of afluid which, for flow through a circular cross sectional conduit isdefined as: ${Re} = \frac{\rho\quad{vd}}{\mu}$

-   -   where: ρ=fluid density;        -   v=fluid mean velocity;        -   d=conduit inside diameter; and        -   μ=fluid viscosity.            It should be noted that the values for density, velocity and            viscosity will vary along the length of the furnace 110. The            maximum Reynolds number in the furnace 110 is typically            attained when the average stream temperature is at a            maximum, because the gas velocity is at a very high value            due to gas expansion when heated.

One problem with operating under flow conditions at a high Reynoldsnumber is that undesirable volatilization of components is much morelikely to occur than in systems having flow characteristics as found inlaboratory-scale systems. The volatilization problem occurs with thepresent invention, because the furnace is typically operated over asubstantial section of the heating zone in a constant wall heat fluxmode, due to limitations in heat transfer capability. This issignificantly different than operation of a furnace at a laboratoryscale, which typically involves operation of most of the heating zone ofthe furnace in a uniform wall temperature mode, because the heating loadis sufficiently small that the system is not heat transfer limited.

With the present invention, it is typically preferred to heat theaerosol stream in the heating zone of the furnace as quickly as possibleto the desired temperature range for particle manufacture. Because offlow characteristics in the furnace and heat transfer limitations,during rapid heating of the aerosol the wall temperature of the furnacecan significantly exceed the maximum average target temperature for thestream. This is a problem because, even though the average streamtemperature may be within the range desired, the wall temperature maybecome so hot that components in the vicinity of the wall are subjectedto temperatures high enough to undesirably volatilize the components.This volatilization near the wall of the furnace can cause formation ofsignificant quantities of ultrafine particles that are outside of thesize range desired.

Therefore, with the present invention, it is preferred that when theflow characteristics in the furnace are such that the Reynolds numberthrough any part of the furnace exceeds 500, more preferably exceeds1,000, and most preferably exceeds 2,000, the maximum wall temperaturein the furnace should be kept at a temperature that is below thetemperature at which a desired component of the final particles wouldexert a vapor pressure not exceeding about 200 millitorr, morepreferably not exceeding about 100 millitorr, and most preferably notexceeding about 50 millitorr. Furthermore, the maximum wall temperaturein the furnace should also be kept below a temperature at which anintermediate component, from which a final component is to be at leastpartially derived, should also have a vapor pressure not exceeding themagnitudes noted for components of the final product.

In addition to maintaining the furnace wall temperature below a levelthat could create volatilization problems, it is also important thatthis not be accomplished at the expense of the desired average streamtemperature. The maximum average stream temperature must be maintainedat a high enough level so that the particles will have a desired highdensity. The maximum average stream temperature should, however,generally be a temperature at which a component in the final particles,or an intermediate component from which a component in the finalparticles is at least partially derived, would exert a vapor pressurenot exceeding about 100 millitorr, preferably not exceeding about 50millitorr, and most preferably not exceeding about 25 millitorr.

So long as the maximum wall temperature and the average streamtemperature are kept below the point at which detrimental volatilizationoccurs, it is generally desirable to heat the stream as fast as possibleand to remove resulting particles from the furnace immediately after themaximum average stream temperature is reached in the furnace. With thepresent invention, the average residence time in the heating zone of thefurnace may typically be maintained at shorter than about 4 seconds,preferably shorter than about 2 seconds, more preferably shorter thanabout 1 second, still more preferably shorter than about 0.5 second, andmost preferably shorter than about 0.2 second.

Another significant issue with respect to operating the process of thepresent invention, which includes high aerosol flow rates, is losswithin the system of materials intended for incorporation into the finalparticulate product. Material losses in the system can be quite high ifthe system is not properly operated. If system losses are too high, theprocess would not be practical for use in the manufacture of particulateproducts of many materials. This has typically not been a majorconsideration with laboratory-scale systems.

One significant potential for loss with the process of the presentinvention is thermophoretic losses that occur when a hot aerosol streamis in the presence of a cooler surface. In that regard, the use of thequench cooler, as previously described, with the process of the presentinvention provides an efficient way to cool the particles withoutunreasonably high thermophoretic losses. There is also, however,significant potential for losses occurring near the end of the furnaceand between the furnace and the cooling unit.

It has been found that thermophoretic losses in the back end of thefurnace can be significantly controlled if the heating zone of thefurnace is operated such that the maximum average stream temperature isnot attained until near the end of the heating zone in the furnace, andat least not until the last third of the heating zone. When the heatingzone includes a plurality of heating sections, the maximum averagestream temperature should ordinarily not occur until at least the lastheating section. Furthermore, the heating zone should typically extendto as close to the exit of the furnace as possible. This is counter toconventional thought which is to typically maintain the exit portion ofthe furnace at a low temperature to avoid having to seal the furnaceoutlet at a high temperature. Such cooling of the exit portion of thefurnace, however, significantly promotes thermophoretic losses.Furthermore, the potential for operating problems that could result inthermophoretic losses at the back end of the furnace are reduced withthe very short residence times in the furnace for the present invention,as discussed previously.

Typically, it would be desirable to instantaneously cool the aerosolupon exiting the furnace. This is not possible. It is possible, however,to make the residence time between the furnace outlet and the coolingunit as short as possible. Furthermore, it is desirable to insulate theaerosol conduit occurring between the furnace exit and the cooling unitentrance. Even more preferred is to insulate that conduit and, even morepreferably, to also heat that conduit so that the wall temperature ofthat conduit is at least as high as the average stream temperature ofthe aerosol stream. Furthermore, it is desirable that the cooling unitoperate in a manner such that the aerosol is quickly cooled in a mannerto prevent thermophoretic losses during cooling. The quench cooler,described previously, is very effective for cooling with low losses.Furthermore, to keep the potential for thermophoretic losses very low,it is preferred that the residence time of the aerosol stream betweenattaining the maximum average stream temperature in the furnace and apoint at which the aerosol has been cooled to an average streamtemperature below about 200° C. is shorter than about 2 seconds, morepreferably shorter than about 1 second, and even more preferably shorterthan about 0.5 second and most preferably shorter than about 0.1 second.In most instances, the maximum average stream temperature attained inthe furnace will be greater than about 800° C. Furthermore, the totalresidence time from the beginning of the heating zone in the furnace toa point at which the average stream temperature is at a temperaturebelow about 200° C. should typically be shorter than about 5 seconds,preferably shorter than about 3 seconds, more preferably shorter thanabout 2 seconds, and most preferably shorter than about 1 second.

Another part of the process with significant potential forthermophoretic losses is after particle cooling until the particles arefinally collected. Proper particle collection is very important toreducing losses within the system. The potential for thermophoreticlosses is significant following particle cooling because the aerosolstream is still at an elevated temperature to prevent detrimentalcondensation of water in the aerosol stream. Therefore, cooler surfacesof particle collection equipment can result in significantthermophoretic losses.

To reduce the potential for thermophoretic losses before the particlesare finally collected, it is important that the transition between thecooling unit and particle collection be as short as possible.Preferably, the output from the quench cooler is immediately sent to aparticle separator, such as a filter unit or a cyclone. In that regard,the total residence time of the aerosol between attaining the maximumaverage stream temperature in the furnace and the final collection ofthe particles is preferably shorter than about 2 seconds, morepreferably shorter than about 1 second, still more preferably shorterthan about 0.5 second and most preferably shorter than about 0.1 second.Furthermore, the residence time between the beginning of the heatingzone in the furnace and final collection of the particles is preferablyshorter than about 6 seconds, more preferably shorter than about 3seconds, even more preferably shorter than about 2 seconds, and mostpreferably shorter than about 1 second. Furthermore, the potential forthermophoretic losses may further be reduced by insulating the conduitsection between the cooling unit and the particle collector and, evenmore preferably, by also insulating around the filter, when a filter isused for particle collection. The potential for losses may be reducedeven further by heating of the conduit section between the cooling unitand the particle collection equipment, so that the internal equipmentsurfaces are at least slightly warmer than the aerosol stream averagestream temperature. Furthermore, when a filter is used for particlecollection, the filter could be heated. For example, insulation could bewrapped around a filter unit, with electric heating inside of theinsulating layer to maintain the walls of the filter unit at a desiredelevated temperature higher than the temperature of filter elements inthe filter unit, thereby reducing thermophoretic particle losses towalls of the filter unit.

Even with careful operation to reduce thermophoretic losses, some losseswill still occur. For example, some particles will inevitably be lost towalls of particle collection equipment, such as the walls of a cycloneor filter housing. One way to reduce these losses, and correspondinglyincrease product yield, is to periodically wash the interior of theparticle collection equipment to remove particles adhering to the sides.In most cases, the wash fluid will be water, unless water would have adetrimental effect on one of the components of the particles. Forexample, the particle collection equipment could include parallelcollection paths. One path could be used for active particle collectionwhile the other is being washed. The wash could include an automatic ormanual flush without disconnecting the equipment. Alternatively, theequipment to be washed could be disconnected to permit access to theinterior of the equipment for a thorough wash. As an alternative tohaving parallel collection paths, the process could simply be shut downoccasionally to permit disconnection of the equipment for washing. Theremoved equipment could be replaced with a clean piece of equipment andthe process could then be resumed while the disconnected equipment isbeing washed.

For example, a cyclone or filter unit could periodically be disconnectedand particles adhering to interior walls could be removed by a waterwash. The particles could then be dried in a low temperature dryer,typically at a temperature of lower than about 50° C.

In one embodiment, wash fluid used to wash particles from the interiorwalls of particle collection equipment includes a surfactant. Some ofthe surfactant will adhere to the surface of the particles. This couldbe advantageous to reduce agglomeration tendency of the particles and toenhance dispersibility of the particles in a thick film pastformulation. The surfactant could be selected for compatibility with thespecific paste formulation anticipated.

Another area for potential losses in the system, and for the occurrenceof potential operating problems, is between the outlet of the aerosolgenerator and the inlet of the furnace. Losses here are not due tothermophoresis, but rather to liquid coming out of the aerosol andimpinging and collecting on conduit and equipment surfaces. Althoughthis loss is undesirable from a material yield standpoint, the loss maybe even more detrimental to other aspects of the process. For example,water collecting on surfaces may release large droplets that can lead tolarge particles that detrimentally contaminate the particulate product.Furthermore, if accumulated liquid reaches the furnace, the liquid cancause excessive temperature gradients within the furnace tube, which cancause furnace tube failure, especially for ceramic tubes. One way toreduce the potential for undesirable liquid buildup in the system is toprovide adequate drains, as previously described. In that regard, it ispreferred that a drain be placed as close as possible to the furnaceinlet to prevent liquid accumulations from reaching the furnace. Thedrain should be placed, however, far enough in advance of the furnaceinlet such that the stream temperature is lower than about 80° C. at thedrain location.

Another way to reduce the potential for undesirable liquid buildup isfor the conduit between the aerosol generator outlet and the furnaceinlet be of a substantially constant cross sectional area andconfiguration. Preferably, the conduit beginning with the aerosolgenerator outlet, passing through the furnace and continuing to at leastthe cooling unit inlet is of a substantially constant cross sectionalarea and geometry.

Another way to reduce the potential for undesirable buildup is to heatat least a portion, and preferably the entire length, of the conduitbetween the aerosol generator and the inlet to the furnace. For example,the conduit could be wrapped with a heating tape to maintain the insidewalls of the conduit at a temperature higher than the temperature of theaerosol. The aerosol would then tend to concentrate toward the center ofthe conduit due to thermophoresis. Fewer aerosol droplets would,therefore, be likely to impinge on conduit walls or other surfacesmaking the transition to the furnace.

Another way to reduce the potential for undesirable liquid buildup is tointroduce a dry gas into the aerosol between the aerosol generator andthe furnace. Referring now to FIG. 49, one embodiment of the process isshown for adding a dry gas 118 to the aerosol 108 before the furnace110. Addition of the dry gas 118 causes vaporization of at least a partof the moisture in the aerosol 108, and preferably substantially all ofthe moisture in the aerosol 108, to form a dried aerosol 119, which isthen introduced into the furnace 110. The dry gas 118 will most often bedry air, although in some instances it may be desirable to use drynitrogen gas or some other dry gas. If sufficient a sufficient quantityof the dry gas 118 is used, the droplets of the aerosol 108 aresubstantially completely dried to beneficially form dried precursorparticles in aerosol form for introduction into the furnace 110, wherethe precursor particles are then pyrolyzed to make a desired particulateproduct. Also, the use of the dry gas 118 typically will reduce thepotential for contact between droplets of the aerosol and the conduitwall, especially in the critical area in the vicinity of the inlet tothe furnace 110. In that regard, a preferred method for introducing thedry gas 118 into the aerosol 108 is from a radial direction into theaerosol 108. For example, equipment of substantially the same design asthe quench cooler, described previously with reference to FIGS. 41-43,could be used, with the aerosol 108 flowing through the interior flowpath of the apparatus and the dry gas 118 being introduced throughperforated wall of the perforated conduit. An alternative to using thedry gas 118 to dry the aerosol 108 would be to use a low temperaturethermal preheater/dryer prior to the furnace 110 to dry the aerosol 108prior to introduction into the furnace 110. This alternative is not,however, preferred.

Still another way to reduce the potential for losses due to liquidaccumulation is to operate the process with equipment configurationssuch that the aerosol stream flows in a vertical direction from theaerosol generator to and through the furnace. For smaller-sizeparticles, those smaller than about 1.5 microns, this vertical flowshould, preferably, be vertically upward. For larger-size particles,such as those larger than about 1.5 microns, the vertical flow ispreferably vertically downward.

Furthermore, with the process of the present invention, the potentialfor system losses is significantly reduced because the total systemresidence time from the outlet of the generator until collection of theparticles is typically shorter than about 10 seconds, preferably shorterthan about 7 seconds, more preferably shorter than about 5 seconds andmost preferably shorter than about 3 seconds.

According to the process of the present invention, and using theapparatus of the present invention, palladium-containing particles of avariety of compositions and for a variety of uses may be made. Many ofthe these particles are producible in novel powder form and include manycompositions believed to have not been heretofore produced by spraypyrolysis processing.

Many applications use palladium-containing powders, and it is usuallydesirable that particles of the powder have one or more of the followingproperties: high purity; high crystallinity; high density; smallparticle size; narrow particle size distribution; spherical morphology;controlled surface chemistry; and reduced agglomeration. The particlesof the present invention are well suited for such applications and maybe used to replace palladium-containing particles of the prior art thatmay currently be used.

Important applications for the palladium-containing particles of thepresent invention are in the manufacture of electronic products.

For example, the particles may be used in electrically conductive orelectrically resistive portions of electronic products. One suchapplication is for the manufacture of electronic products includingpalladium-containing thick film features that are active in the functionof the product when used. For example, the particles may be used to makecapacitor electrodes for chip capacitor designs, includingsupercapacitors, and especially for multi-layer capacitors. Also, theparticles may be used in the manufacture of resistive films, such as maybe used in serpentine meander circuits in surge protector resistorsystems.

Another application is to make thick film metallized terminations onelectronic components. These terminations are common on surface mountcomponents to permit easy interconnection of the component into anelectrical circuit. For example, the component may be mounted on acircuit board by soldering to the metallized terminations.

Another application is to make thick film electrical interconnectionswithin electronic products. For example, the particles may be used tomake electrical interconnections within a multi-chip module or on acircuit board.

Many other applications are also possible for the palladium-containingpowder of the present invention, some of which are discussed below.

The powder of the present invention is of a high quality that isdesirable for many applications. The palladium-containing particles ofthe powder have a small average particle size. Although the preferredaverage size of the particles will vary according to the particularapplication, the number average particle size is typically in a rangehaving a lower limit of about 0.1 micron, preferably about 0.2 micron,more preferably about 0.5 micron and most preferably about 0.8 micron;and having an upper limit of about 5 microns, preferably about 3 micronsand more preferably about 2 microns.

Another indication of the high quality of the powder is based on theweight average particle size and size distribution ofpalladium-containing particles on a weight basis. This is because aweight basis is more sensitive to the presence of oversize particles,which are usually more detrimental in the powder of the presentinvention than undersize particles, although both are generallyundesirable. In that regard, it is preferred that thepalladium-containing powder has a weight average particle size in arange having a lower limit of about 0.1 micron, preferably about 0.2micron, more preferably about 0.5 micron and most preferably about 0.8micron; and having an upper limit of about 5 microns, preferably about 3microns and more preferably about 2 microns.

The palladium-containing particles of the powder also typically have anarrow particle size distribution, such that the majority of particlesare substantially the same size. Preferably, at least about 75 percentby number, more preferably at least about 85 percent by number, evenmore preferably at least about 90 percent by number and most preferablyat least about 95 percent by number of the particles are smaller thantwice the number average particle size. Thus, when the number averageparticle size is about 2 microns it is preferred, for example, that atleast about 75 number percent of the particles are smaller than about 4microns. Further, it is preferred that at least about 75 number percent,more preferably at least about 85 number percent, even more preferablyat least about 90 number percent and most preferably at least about 95number percent of the particles are smaller than about 1.5 times thenumber average particle size. Thus, when the number average particlesize is about 2 microns, it is preferred, for example, that at leastabout 75 number percent of the particles are smaller than about 3microns.

Furthermore, the palladium-containing particles of the powder have aparticle size distribution such that preferably at least about 75 weightpercent of the particles, more preferably at least about 85 weightpercent of the particles, still more preferably at least about 90 weightpercent of the particles, and most preferably at least about 95 weightpercent of the particles are smaller than twice the weight averageparticle size; and even more preferably smaller than about 1.5 times theweight average particle size, in a manner as stated previously with theparticle size distribution with respect to number average particle size.

The palladium-containing particles of the powder typically have a highdegree of purity, and it is preferred that the particles include lessthan about 0.1 weight percent impurities and more preferably less thanabout 0.01 weight percent impurities. Most preferably the particles aresubstantially free of contaminants. The purity of the particles of thepresent invention is one major advantage over particles manufactured byliquid phase routes. The powders of the present invention aresubstantially free of contaminants, and particularly surfactants andother organic materials that are often left as residue in powders madeby liquid routes. The substantial absence of residual surfactants andother organics is important in making pastes for thick filmapplications, because the surfactants, or other organics, are oftenincompatible with other paste ingredients. Also, the presence of suchorganics or surfactants can sometimes cause complications during filmbake-out, and can impair film performance, especially if high electricalconductivity is desired.

The palladium-containing particles of the powder also typically have avery high density. Preferably, the powder exhibits a particle density,as measured by helium pycnometry, of at least about 80 percent oftheoretical, more preferably at least about 90 percent of theoreticaldensity and even more preferably at least about 95 percent of thetheoretical density. Most preferably, the powders exhibit a particledensity, as measured by helium pycnometry, of at least about 99 percentof the theoretical density. The theoretical density is that density thatthe particles would have assuming zero pore volume within the particles.

The palladium-containing particles of the powder are also typicallysubstantially spheroidal in shape. A high degree of sphericity isadvantageous because the particles are able to be dispersed morereadily, imparting advantageous rheological characteristics to pasteformulations, including the particles. The powders of the presentinvention are substantially non-agglomerated and have gooddispersibility in a variety of liquid vehicles used in thick film pasteformulations.

The palladium-containing particles of the powder also typically havegood crystallinity. The metallic phase including the palladium typicallypreferably includes a mean crystallite size of larger than about 50nanometers, and more preferably larger than about 100 nanometers. Themetallic phase is, advantageously, often polycrystalline, but with theserelatively large mean crystallite sizes.

Although the palladium in the particles may be in any form, including inthe form of palladium oxide, it is preferred that substantially all ofthe palladium be in elemental form in a metallic phase. This metallicphase may be a palladium-containing alloy or may consist essentially ofonly palladium. Preferred alloy metals include silver (Ag), nickel (Ni),copper (Cu), platinum (Pt), molybdenum (Mo), tungsten (W), tantalum(Ta), aluminum (Al), gold (Au), indium (In), lead (Pb), tin (Sn),bismuth (Bi) and the like. When alloyed with one or more other metals,palladium will typically comprise from about 1 weight percent to about99 weight percent. In one embodiment, the alloy includes less than about30 weight percent palladium, while in another embodiment the alloyincludes greater than about 50 weight percent palladium, or even greaterthan about 70 weight percent palladium. The desired composition of analloy will vary depending upon the specific application.

One preferred alloy is with silver. Particles including an Ag:Pd alloyare particularly preferred for use in making internal electrodes ofmulti-layer capacitors and for high performance applications forelectrical interconnections in multi-chip modules. In that regard,however, it has traditionally been difficult to prepare, in a highquality powder form, a true alloy between palladium and silver, i.e.,homogeneous at the molecular level. This has also been found to be thecase with spray pyrolysis unless the conditions of particle manufactureare carefully controlled.

The particular alloy composition will depend upon the specificapplication for which the particles are intended. For most applications,however, silver/palladium alloys of the present invention preferablyinclude from about 30 weight percent to about 90 weight percent silverand from about 10 weight percent to about 70 weight percent palladium,with more specific alloys in that preferred range being more preferredfor different applications.

For electronic components, including multi-layer capacitors andmulti-chip modules, manufactured in low fire processes (with firingtemperatures generally lower than about 700° C.), silver-rich alloyswith palladium are preferred. Preferred alloys for low fire processesinclude from about 10 weight percent to about 30 weight percentpalladium and from about 70 weight percent to about 90 weight percentsilver. Particularly preferred for low fire processes are alloys havinga weight ratio of Pd:Ag of about 20:80 and 30:70.

For electronic components, such as multi-layer capacitors and multi-chipmodules, manufactured in high fire processes (with firing temperaturesgenerally above about 800° C.), palladium-rich alloys are preferred.Preferred alloys for high fire processes include from about 50 weightpercent to about 70 weight percent palladium and from about 30 weightpercent to about 50 weight percent silver. Particularly preferred forhigh fire processes are alloys having a weight ratio of Pd:Ag of about50:50 and 70:30.

According to the present invention, it has, surprisingly, been foundthat certain spray pyrolysis manufacturing conditions are particularlyconducive to preparation of a true alloy between silver and palladium.

According to the process of the present invention, when making particlesincluding a silver/palladium alloy, or for making pure palladium, themaximum average stream temperature of the aerosol the furnace shouldordinarily be in a range with a lower limit of about 900° C., preferablyabout 950° C., more preferably about 975° C., and even more preferablyabout 1000° C.; and with an upper limit of about 1300° C., preferablyabout 1200° C., more preferably about 1150° C., even more preferablyabout 1100° C., and still more preferably about 1050° C. If thetemperature is too low, the particles do not adequately densify andsignificant porosity in the particles can result. As the temperatureexceeds about 1100° C., however, the vapor pressure of silver can becomesignificant, and significant vaporization of silver during particlemanufacture appears to occur, which can result in the production ofundesirable ultrafines of silver. This problem becomes particularlypronounced above about 1200° C. Such vaporization can result insignificant segregation of silver and defeat the objective of preparingan alloy between silver and palladium. The presence of a segregated,unalloyed silver phase in a powder is undesirable because of the highmobility of silver in microelectronic devices when the silver is in anunalloyed state and because the palladium will tend to be moresusceptible to undesirable oxidation during manufacture ofmicroelectronic devices as more of the silver segregates.

When making alloys of palladium and silver, a particularly preferredrange for the maximum average stream temperature in the furnace is fromabout 950° C. to about 1100° C. Even more preferred is a temperaturerange of from about 975° C. to about 1050° C., with a manufacturetemperature of approximately 1,000° C. being particularly preferred.

Furthermore, it has been found that the vapor pressure of silver shouldgenerally be maintained at a relatively low level during processing toavoid the production of ultrafines that degrade the quality of theparticulate product. This is especially important when preparing alloys.When the maximum average stream temperature of the furnace is maintainedat a temperature low enough to prevent the vapor pressure of the silverfrom exceeding about 100 millitorr, and more preferably about 50millitorr, it has been most surprisingly found that the resultingparticles are generally of a higher quality, with a reduced amount ofsilver fines.

The palladium-containing particles may include only a single materialphase, which would include the palladium. Alternatively, thepalladium-containing particles may be multi-phase, or composite,particles. In multi-phase particles, the palladium is present in a firstmaterial phase. The particles also include a second material phase thatis different than the first material phase. The multi-phase particlesmay, however, include more than two material phases.

Single phase particles will typically consist essentially of a singlemetallic phase of palladium metal or a palladium-containing alloy.Multi-phase particles also typically include a palladium-containingmetallic phase and at least one other phase. Besides thepalladium-containing metallic phase, the other phases that may bepresent are other metallic phases, that are preferably substantiallyfree of palladium, or nonmetallic phases, that are also preferablysubstantially free of palladium.

For many applications, whether single phase or multi-phase particles areused, the palladium-containing metallic phase will frequently comprisegreater than about 50 weight percent palladium, preferably greater thanabout 60 weight percent palladium, more preferably greater than about 70weight percent palladium. Examples of applications when an essentiallypure palladium or palladium-rich metallic phase is desirable includeinternal electrodes for multi-layer capacitors manufactured in a processinvolving a high firing temperature (such as higher than about 800° C.).It should be recognized, however, that many applications exist wherepalladium will be the minor component in the alloy. For example, themetallic phase of some particles may include 30 weight percent or lessof palladium, especially for multi-layer capacitors manufactured in aprocess involving a low firing temperature (such as lower than about700° C.). Other examples where an alloy having palladium as the minorcomponent include thick film metallized terminations forsurface-mountain electronic components, conductive lines for printedcircuits, horizontal interconnects in multi-chip modules and verticalinterconnects in multi-chip modules.

Multi-phase particles may be desirable for a number of reasons,including: (1) a reduction in the amount of the expensive palladium thatis used in the particle to provide electrical conductivity byincorporating a second material phase that is a less expensive fillermaterial; (2) to improve flowability of the particles in a paste and toimprove resistance of particles to deformations; (3) to modify physicalproperties of the particles for improved compatibility with a substratesupporting a conductive film made using the particles, includingmodifications of the thermal coefficient of linear expansion,modification of sintering/densification characteristics, andmodification of surface energy to alter wetability of the particles; and(4) to modify electrical or dielectric properties for customizedelectronic components. Some examples of uses of the multi-phase,palladium-containing particles include use as catalysts or catalyticsupports and as particles in paste formulations used in thick filmapplications, including manufacture of multi-layer capacitors,multi-chip components, super capacitors and other electronic components.

In the case of multi-phase particles, the particles include at least afirst material phase and a second material phase. Additional materialphases may be present, if desired. The first material phase includespalladium, and is typically an electrically conductive metallic phase,with the palladium being in the form of substantially pure palladium oran alloy with one or more other metal. The second material phase, whichis different than the first material phase, is typically substantiallyfree of palladium.

The second material phase may be a metallic phase. When the secondmaterial phase is a metallic phase, it may consist essentially of asingle metal, or may include an alloy of two or more metals. Examples ofsome metals that may be included in the second material phase includesilver, nickel, copper, platinum, molybdenum, tungsten, tantalum,aluminum, gold, indium, lead, tin, bismuth, and the like.

For most applications, however, the second material phase will benonmetallic, in which case the second material phase will also typicallynot be electrically conductive. Preferred in a nonmetallic secondmaterial phase are a variety of ceramic materials, glass materials orother materials that would alter the sintering characteristics of theparticles. Control of sintering characteristics of the particles isparticularly important when the particles are to be used in a paste formanufacture of a palladium-containing film on a substrate including aceramic layer, which is typically dielectric, to more closely match withthe sintering and shrinkage characteristics of the powder particles withthose of the substrate, thereby reducing the occurrence of problems suchas film cracking and delamination. This is particularly important whenlayers are to be cofired.

The second material phase may include an oxide material, such as oxidesof zinc, tin, barium, molybdenum, manganese, vanadium, niobium,tantalum, tungsten, iron, silver, chromium, cobalt, nickel, copper,yttrium, iridium, beryllium, silicon, zirconium, aluminum, bismuth,magnesium, thorium and gadolinium. Some preferred oxides are silica,alumina, titania, zirconia, yttria, and oxides of copper, bismuth andtin. Another preferred group of oxides includes borates, titanates,silicates (including borosilicates and aluminosilicates), aluminates,niobates, zirconates and tantalates. Examples include mullite,cordierite, barium titanate, neodymium titanate, magnesium titanate,calcium titanate, strontium titanate and lead titanate. Additionalmaterials that could be used as the second material phase include glassmaterials, such as glass frits and glazes. Particularly preferred aresecond material phases including titanates, and especially including atitanate of one or more of barium, strontium, neodymium, calcium,magnesium and lead. The titanate may be of a single metal or may be amixed metal titanate, such as, for example Ba_(x)Sr_(1-x)TiO₃.Furthermore, a variety of other ceramic materials may be used in thesecond material phase, such as carbides and nitrides, including siliconnitride. Also, porcelain could be used in the second material phase.

The multi-phase particles of the present invention may typically be usedin place of single phase metallic particles for most application, solong as the proportion of the second material phase making up theparticles is small enough not to be detrimental to the application.Often, however, the use of multi-phase particles significantly enhancesthe performance of films made using the particles relative to the use ofsingle phase metallic particles.

One use for the multi-phase particles of the present invention is toform a film including palladium in a metallic phase, often electricallyconductive, adjacent to a layer of nonmetallic material, oftendielectric. In that case, the multi-phase particles will typicallyinclude in the second phase a nonmetallic material that enhancessuitability for use with the nonmetallic layer, resulting in improvedcompatibility and bonding between the nonmetallic layer and the filmincluding the metallic phase. For example, if the palladium-containingfilm is a conductive electrode line on a silicon-based photovoltaiclayer, the palladium-containing particles may include a glass frit inthe second material phase of the palladium-containing particles. Formany of these applications, the multi-phase palladium-containingparticles will include in the second material phase a nonmetallicmaterial that is also present in an adjacent nonmetallic layer. Thus,when the nonmetallic layer is a dielectric material that is also presentin the dielectric layer. When the nonmetallic layer is a ceramic layer,for example, the multi-phase particles could include in the second phasea ceramic material that is also present in the ceramic layer. As onespecific example, titanate materials are often used in the dielectriclayers of multi-layer capacitors, and the palladium-containing particlesused to make internal electrodes for the multi-layer capacitor couldinclude in the second material phase the same titanate that is presentin the dielectric layers.

Generally, for applications involving the use of multi-phase particlesto form a metallic electrically conductive phase adjacent a dielectriclayer, the second material phase of the particles typically comprisesless than about 30 weight percent of the particles, preferably less thanabout 20 weight percent of the particles, and more preferably less thanabout 10 weight percent of the particles.

Multiphase particles having a very low content of the second materialphase are generally preferred when the particles will be used to makeelectrically conductive features, because the second material phase istypically dielectric and reduces electrical conductivity. In manyinstances, therefore, and especially those including silica, alumina ora titanate as the second material phase, the second material phasetypically comprises less than about 10 weight percent of the particles,more preferably less than about 5 weight percent of the particles, andeven more preferably less than about 2 weight percent of the particles;but the second material phase will typically be at least about 0.5weight percent of the particles. In this way, enhanced compatibilitybetween the dielectric layer and the electrically conductive film may beaccomplished without significant detrimental impact to electricalconductivity. also, the use of the multiphase particles to makeelectrically conductive films will typically result in improved adhesionfor better bonding with the dielectric layer, thereby reducing thepotential for delaminations.

A number of different variations of the process of the present inventionare possible for making the multi-phase particles. In one embodiment, apalladium-containing precursor for the first material phase and a secondprecursor for the second material phase may both be included in theliquid feed 102 (referring back to FIGS. 1-47 and the discussionrelating thereto). In such a case, both precursors could be in solutionin a flowable liquid of the liquid feed 102. Alternatively, one or bothof the precursors could be particles suspended in the flowable liquid.Also, it is possible that the liquid feed 102 could include more thantwo precursors for the multi-phase particles. In another embodiment, thepalladium-containing precursor could initially be in the liquid feed102, which is then processed in aerosol form in the furnace 110 toprepare palladium-containing precursor particles. The precursorparticles are then coated with the second material phase in a separatestep, in a manner similar to that described previously with reference toFIG. 45. This two-step process of initially preparingpalladium-containing precursor particles and then coating the precursorparticles on the fly in an aerosol state is particularly advantageousbecause problems are avoided that are encountered in particlemanufacture procedures, such as liquid route precipitation, in whichprecursor particles would have to be collected and then redispersedprior to coating. Not only is collection and redispersion cumbersome,but problems are often encountered due to particle agglomeration, whichis avoided with the on-the-fly coating of the present invention.Avoidance of particle agglomeration is very important when a uniformparticle coating is desired.

As noted previously, the multi-phase particles of the present inventionmay include a variety of particle morphologies. With reference again toFIG. 47, the multi-phase particles may include an intimate mixture ofthe first material phase and the second material phase, as in themulti-phase particle 500. Typically, with such an intimate mixture, thefirst material phase is a continuous phase throughout which the secondmaterial phase is dispersed. Another possible morphology is for thefirst material phase to be in the form of a large core that is coveredby a thin coating layer of the second material phase, as shown forparticles 502 and 504 in FIG. 47. Whether such coatings form a smoothcoating, such as shown in particle 502, or a rough and bumpy coating,such as shown in particle 504, will depend upon the wetabilitycharacteristics of the first and second material phases and theconditions under which the materials are processed, and especially theprocessing temperature. For example, in gas-to-particle conversionprocesses, higher temperatures during the coating operation tends toresult in smoother, more continuous coatings. The multi-phase particlescould also include a small core of one material phase surrounded by athick layer of the other material phase, as shown for particle 506.Also, the first and second material phase could completely segregate ina manner shown for particle 508. Furthermore, the multi-phase particlesare not limited to two material phases. For example, particle 510 inFIG. 47 shows a multi-phase particle including a core of second materialphase domains dispersed in a matrix of the first material phase, andwith the core being coated by a third material phase.

With continued reference to FIG. 47, it should be noted that the firstmaterial phase and the second material phase could constitute any of thephases in particles 500, 502, 504, 506, 508 and 510. For mostapplications, however, the first material phase, which includespalladium, will be the more abundant phase, and the second materialphase will be the less abundant phase.

In the case of coated particles, the second material phase will often bein the form of a coating around a core including the first materialphase. In the case of catalyst materials, however, the first materialphase may be a coating or a disperse phase on the surface of a supportof the second material phase. For particles including intimate mixturesof the two phases, the first material phase will typically be thecontinuous phase and the second material phase will typically be thedisperse phase.

For most applications, the multi-phase particles will include greaterthan about 50 weight percent of the first material phase, morepreferably greater than about 60 weight percent of the first materialphase, even more preferably greater than about 70 weight percent of thefirst material phase and most preferably greater than about 80 weightpercent of the first material phase. In the case of multi-phaseparticles including thin coating layers of the second material phase,the first material phase may comprise 90 weight percent or more of theparticles. Conversely, the second material phase typically will compriseless than about 50 weight percent of the multi-phase particles,preferably less than about 40 weight percent, more preferably less thanabout 30 weight percent and even more preferably less than about 20weight percent. In the case of thin coatings of the second materialphase, the second material phase may comprise 10 percent or less of theparticles. Even in the case of coated particles, however, the secondmaterial phase will typically comprise greater than about 0.5 weightpercent, and preferably greater than about 1 weight percent, of theparticles.

Because most applications for multi-phase particles of the presentinvention include the use of either a particle including the firstmaterial phase in a large core surrounded by a thin coating of thesecond material phase or an intimate mixture of the first material phaseas a continuous phase with the second material as a disperse phase,those particular situations will now be discussed below in greaterdetail.

One preferred class of multi-phase particles are coated particles inwhich the first material phase forms a core and the second materialphase forms a thin coating layer about the outer surface of the core.The second material phase may include any of the materials previouslylisted as being suitable for the second material phase.

With the present invention, the coating including the second materialphase may be made as a relatively uniform layer that substantiallyentirely covers the core of the first material phase. One method formaking multi-phase particles including the second material phase as auniform coating is as described previously with reference to FIG. 45. Inthat regard, the second material phase is typically formed on aprecursor particle, which includes the first material phase, bytechniques as previously described. A preferred coating technique isCVD. CVD is a well known deposition technique in which a precursor forthe second material phase is reacted in the vapor phase to form thesecond material phase. Generally, precursors for CVD aremetal-containing compounds, for example, inorganic compounds, metalorganics and organometallics. Examples of some vapor phase precursorsfor CVD of inorganic coatings include silanes, metal formates, metalacetates, metal oxalates, metal carboxylates, metal alkyls, metal aryls,metal alkoxides, metal ketonates (especially beta-diketonates), metalamides, metal hydrides, metal oxyhalides, and metal halides (especiallymetal chlorides and metal bromides). For example, to deposit a coatingof silica, a vaporous silane precursor, such as silicon tetrachloride,may be decomposed and converted to silica at elevated temperature in thepresence of oxygen or water vapor, with the silica then depositing onthe surface of palladium-containing precursor particles.

Typically, a coating deposited by CVD or by PVD will result in anaverage coating thickness of from about 10 nanometers to about 200nanometers. Preferred coatings have an average thickness of thinner thanabout 100 nanometers, more preferably thinner than about 50 nanometersand most preferably thinner than about 25 nanometers.

Applications for coated multi-phase particles include the manufacture ofelectrically conductive films for electronic devices, such asmulti-layer capacitors and multi-chip modules. In the case of manycoatings such as silica, the coating is useful to beneficially alter thesintering and/or shrinkage characteristics of the particle for improvedcompatibility with a ceramic substrate.

Another way to make coated multi-phase particles is to provideprecursors for both the first material phase and the second materialphase in the feed liquid 102 (as described with respect to FIGS. 1-49).As noted previously, each precursor in the feed liquid 102 could beeither in the liquid phase, e.g., in solution in a flowable liquid, orin the form of particles suspended by the flowable liquid. Themulti-phase particles would then form in the furnace 110 as liquid isremoved from aerosol droplets. It should be noted that, in the case ofmultiple phases forming simultaneously in the furnace, the differentphases are typically initially formed as an intimate mixture of thephases. Generally, higher processing temperatures and longer residencetimes will result in redistribution of the material phases to thedesired morphology of a coating of one material phase about a core ofthe other material phase, assuming that the two material phases have theproper wetability characteristics. Alternatively, it is possible thatredistribution of the phases could result in complete segregation of thephases, as shown by the multi-phase particles 508 in FIG. 47. Whenredistribution of the material phases is desired to form a coatedparticle morphology, a processing embodiment such as that describedpreviously with reference to FIG. 46 may be advantageous.

When making coated particles with precursors for both the first materialphase and the second material phase in the liquid feed 102, a firstprecursor for the palladium-containing first material phase couldcomprise preformed palladium-containing particles to be coated. Theprecursor for the second material phase could also be in particulateform, or could be in solution in a liquid phase. For example, a solubleprecursor, such as from dissolution of a metal alkoxide could be used asa precursor for the second material phase. In the case of metalalkoxides, it should be recognized that in aqueous solution thedissolved metal alkoxide usually reacts to form other solublecomponents, which will function as a soluble precursor. This could bethe case in the preparation of particles including titania or alumina asthe second material phase. In the case of silica as the second materialphase, the precursor will typically be small silica particles, which arepreferably of colloidal size, or silica dissolved in solution.

The manufacture of multi-phase particles with an intimately mixedmorphology for the different material phases is typically accomplishedby initially including a precursor for both the first material phase andthe second material phase in a liquid feed 102, as previously described.As noted, the process may be substantially the same as the process usedto prepare particles with a coating morphology, except the processingconditions may be altered so that the material phases do notredistribute, and are instead retained in an intimately mixed state.Generally, lower operating temperatures in the furnace 110 and shorterresidence times, with rapid particle cooling, promote an intimatemixture of the phases.

Multi-phase particles of an intimately mixed morphology are particularlyuseful for modifying sintering/densification temperatures of theparticle, reducing shrinkage that occurs during firing in thick filmapplications, and modifying the electrical or other properties of theparticle for special applications.

As noted previously, the palladium-containing particles of the presentinvention may be used in the manufacture of a variety of products, whichproducts are also within the scope of the present invention, as are themethods for making those products. The powders typically may includepalladium-containing particles of only a single phase and/or includingany of the multi-phase particles previously discussed. The use ofmulti-phase particles, however, is often preferred. Furthermore, for allof the applications discussed below, a powder having any combination ofthe characteristics of size, size distribution, sphericity,crystallinity and any other characteristic described herein for thepowders of the present invention. In general, high levels of sphericityand crystallinity and with a narrow size distribution are preferred.Although average particle sizes may be any convenient size describedpreviously. For some applications, particular particle size ranges aremore preferred, as noted when applicable.

The palladium-containing powders of the present invention may be used inthe manufacture of a variety of electronic products, with the powderstypically being used to form electrically conductive features, ofvarying resistivity, in the products.

FIG. 50 shows one embodiment of a multi-layer capacitor 400 made usingthe particles of the present invention. The capacitor includes astructure of stacked layers, including electrically conductive internalelectrode layers 404 separated by dielectric layers 402. The dielectriclayers may include a variety of dielectric materials, mostly ceramics.

The internal electrode layers 404 are made using a palladium-containingpowder of the present invention. The capacitor also includes metallizedterminations 406 at either end of the capacitor that serve as electricalcontacts for interconnecting the capacitors in an electrical circuitwhen the capacitor is used. The terminations 406 are typically made froma paste including particles of an electrically conductive metal. Theterminations 406 may also be made from a palladium-containing powder ofthe present invention, as is discussed in greater detail below. Thediscussion here focuses on use of the particles of the present inventionfor making the internal electrode layers 404.

There are a variety of structures and configurations known in the artfor multi-layer capacitors, any of which may be made using apalladium-containing powder of the present invention. The manufacturingtechniques are well known in the art and need not be significantlyaltered to accommodate use of the palladium-containing powders of thepresent invention. Furthermore, it should be appreciated that although amulti-layer capacitor is discussed as an example, the same principlesapply to other capacitor designs, including supercapacitors, thatinclude palladium-containing electrodes made by thick film techniques.

The palladium-containing powder, however, used to make the internalelectrode layers 404 preferably has a weight average particle size in arange having a lower limit of preferably about 0.2 micron and morepreferably about 0.3 micron; and an upper limit of preferably about 1micron, more preferably about 0.8 micron and even more preferably about0.6 micron. Particularly preferred is a weight average particle size ina range of from about 0.3 micron to about 0.6 micron.

The palladium-containing powders of the present invention used to makethe internal electrode layers 406 may include the palladium in singlephase particles having only a palladium-containing metallic phase, whichis frequently either substantially pure palladium or a silver-palladiumalloy. Alternatively, in a preferred embodiment, the internal electrodelayers 404 are made using, at least in part, palladium-containingmulti-phase particles that include at least a second material phase, aspreviously discussed, that modifies the sintering and/or densificationproperties of the palladium-containing particles for enhanced bondingand compatibility with the dielectric layers 402. Although the secondmaterial phase could include any of the materials previously listed, thesecond material phase preferably includes a dielectric material that isalso present in the dielectric layers 404. Therefore, if the dielectriclayers 402 include a titanate, as previously discussed, then the secondphase of the palladium-containing particles used to make the internalelectrode layers 404 would also include at least some of that sametitanate. When the stacked layers are then cofired, a bonding zonedevelops at the interface between adjacent ones of the internalelectrode layers 404 and the dielectric layers 402, with enhancedbonding in the bonding zone due to the inclusion of the dielectricmaterial in the second phase of the multi-phase particles. The use ofthe multi-phase particles of the present invention to make the internalelectrode layers 410 provides a significant performance advantage interms of bonding and compatibility with the dielectric layers 402,relative to the use of a simple mixture of dielectric particles andmetallic palladium-containing particles.

When using multi-phase particles to make the internal electrode layers404, it should be recognized that a paste preparation for making theinternal electrode layers 404 could include only the multi-phaseparticles of the present invention, provided that the quantity of theelectrically conductive first material phase is sufficient to provide adesired degree of electrical conductivity. Alternatively, a paste fromwhich the internal electrode layers 404 are made could include a mixtureof the multi-phase particles and single-phase particles including only apalladium-containing electrically conductive metallic phase. Also,particles of any other composition could be added to the paste as deemeddesirable. Referring now to FIG. 51, a preferred embodiment of thestacked structure of the multi-layered capacitor 400 is shown.Interposed between the dielectric layers 402 are the electricallyconductive internal electrode layers 404 made with palladium-containingpowder of the present invention. In the structure shown in FIG. 50, theinternal electrode layers 404 each include only a singlepalladium-containing film. The single palladium-containing film is madeusing single or multi-phase particles, or mixtures of particles.

In another embodiment, however, the internal electrode layers 404 couldinclude sublayers to provide a transition from the dielectric materialof the dielectric layers 402 to the metallic material of the internalelectrode layers 404. FIG. 52 shows stacked layers of one embodiment ofa multi-layer capacitor including internal electrode layers 404comprised of sublayers for gradation of the metallic content in theinternal electrode layers 404. The internal electrode layers 404 eachinclude two first sublayers 408 and one second sublayer 410. The firstsublayers 408 are leaner in a palladium-containing, electricallyconductive metallic phase than the second sublayers 410. The firstsublayers 408 act as interlayers to enhance bonding and compatibilitybetween the dielectric layers 402 and the more electricity conductivesecond sublayers 410. For example, the first sublayers 408 could be madefrom multi-phase particles, as previously discussed, while the secondsublayers 410 are made from single-phase metallic particles. As analternative, the first sublayers 408 could be made from multi-phaseparticles having a higher content of dielectric material and the secondsublayers 410 could be made from multi-phase particles having a lowercontent of dielectric material. As yet another alternative, the firstsublayers 408 could be made from a mixture of multi-phase andsingle-phase particles including a higher proportion and the secondsublayers 410 could be made from a mixture of multi-phase particlesincluding smaller proportion of the multi-phase particles. As will beappreciated, other alternatives are also possible for obtaining acompositional gradient between sublayers 408 and 410 of the internalelectrode layers 404. Also, more than two sublayers could be used, orthe internal electrode layers could include a continuous gradation ofcomposition.

Another application for the palladium-containing powders of the presentinvention is for use in making metallized terminations on electroniccomponents. Through the metallized terminations, the electroniccomponents may be interconnected into an electrical circuit. Theseterminations are typically on components adopted for surface mounting,such as on a circuit board, via a solder connection between themetallized terminations of the component and metallized portions of thecircuit board. The particles of the present invention may be used in themanufacture of terminations for a variety of electronic components,including capacitors (including multi-layer capacitors, supercapacitors,etc.), inductors (including multi-layer inductors), resistors, fuses,resonators, trimmers, potentiometers, thermisters, varistors and thewrap-around terminations for hybrid circuits. These components arefrequently ceramic chip components.

By way of example, metallized terminations are now discussed withreference again to FIG. 50. Although FIG. 50 specifically concerns amulti-layer capacitor, the discussion concerning terminations appliesequally to metallized terminations on other electronic components. Asshown in FIG. 50, the multi-layer capacitor 400 includes metallizedterminations 406 on opposite ends of the multi-layer capacitor 400.These metallized terminations 406 may advantageously be used to solderthe component for surface mounting, such as on a circuit board, whichmay be a single-layer board a multi-layer package.

With continued reference to FIG. 50, the metallized terminations 406include at least one electrically-conductive film made usingpalladium-containing powder of the present invention. The powder willoften include only single-phase particles; however, multi-phaseparticles may be used to modify sintering and densificationcharacteristics of the film for better compatibility with a dielectricmaterial of the component, if desired. To make the metallizedterminations 406, the particles are applied from a paste or slurryincluding the particles by any appropriate technique, such as by dipcoating, spraying, brushing or printing. The terminations 406 are thenfired to develop a dense, electrically conductive film. Although anyconvenient firing temperature may be used, firing temperatures are oftenin the range of from about 300° C. to about 700° C. For making themetallized terminations 406, the palladium-containing powder willpreferably have a weight average particle size of from about 0.5 micronto about 2 microns. Furthermore, the metallic phase of the particles ofthe present invention will often be an alloy of palladium, andespecially with silver. Also, as with all of the applications for thepalladium-containing powders, an additional metal, such as silver, couldbe present in a separate phase in the particles, if desired. This is analternative to providing another metal in an alloy with palladium.

With continued reference to FIG. 50, the terminations 406 may compriseonly a single layer made using the palladium-containing particles of thepresent invention. Alternatively, however, the terminations 406 mayinclude multiple metallic layers for enhanced performance, andespecially for enhanced solderability. For example, one metallizedtermination structure that may be used includes a base film of asilver-rich palladium alloy, which may be made using powder of thepresent invention, a nickel barrier film overcoating the base film and atin plating film overcoating the nickel barrier film.

Another important application of the palladium-containing powders of thepresent invention is to make circuit interconnections in electronicproducts, and particularly in circuit boards. Such a circuit board maycomprise a single layer structure or may comprise a multi-layerstructure. Preferred circuit boards are those including one or moredielectric layers of low temperature cofired ceramic on and/or throughwhich electrical interconnections are made using palladium-containingpowder of the present invention. By low temperature cofired ceramic, itis meant ceramic compositions fireable at a relatively low temperaturein a single firing operation in which thick film metallizations are alsofired. Firing temperatures for such low temperature cofirings arefrequently at a temperature of lower than about 800° C. For cofiringoperations, powders of the present invention including multi-phaseparticles may be advantageously used, although single-phase particlescould be used if desired. In the case of a single layer board, theparticles of the present invention are useful for making conductivelines for interconnecting different portions of the board wherecomponents may be mounted. For multi-layer packages, thepalladium-containing powders of the present invention may advantageouslybe used for both horizontal interconnections and verticalinterconnections.

The boards may come in a variety of configurations, but all are designedso that electronic components may be mounted on and bound to a surfaceof the board in a manner to provide desired electrical interconnectionbetween the mounted components. The electronic components could be anycomponents, for example capacitors, inductors, resistors and integratedcircuits. Electronic components may be mounted on only one side of theboard or may be mounted on both sides of the board. Multi-layer packagesmay also have electronic components embedded between dielectric layers.

Single-layer boards (those having only a single dielectric layer) areoften referred to as hybrid circuit boards, whereas boards having aplurality of stacked dielectric layers are referred to by a variety ofnames, including multi-layer packages, multi-layer ceramic packages, andmulti-chip modules.

With reference now to FIG. 53, one embodiment of a multi-layer ceramicpackage 420 is shown, including examples of several interconnectionsthat may be made using palladium-containing powder of the presentinvention. As seen in FIG. 53, the multi-layer ceramic package 420includes three stacked dielectric layers 422, which are typically madefrom low temperature cofired ceramic materials. Shown in FIG. 53 areconductor lines 424, to provide horizontal interconnections in themulti-layer ceramic package 420. Also shown is a filled via 426providing a vertical interconnection that extends across the entirethickness of the stacked structure. The filled via 426 is covered with ametallized cover pad 428 which enhances the interconnection between thefilled via 426 and one of the conductor lines 424. Also shown is a blindvia 430, which provides vertical interconnection only across one of thedielectric layers 422. Also shown is an edge metallization 432, which isa metallic film providing a vertical interconnection across the entirethickness at the edge of the multi-layer ceramic package 420, andproviding a connection to one of the conducting lines 424.

It should be recognized that the multi-layer ceramic package 420 shownin FIG. 53 is a very simplified structure showing some features that maybe present in a multi-layer ceramic package. It will be appreciated thatmulti-layer ceramic packages may be constructed having any number ofdielectric layers and having a wide variety of vertical and horizontalinterconnections. For example, a multi-layer ceramic package may includecoated vias, in which the bore of the via is coated with a conductivefilm.

Any of the horizontal and vertical interconnections in the multi-layerceramic package 420, including the cover pad 428, may be advantageouslymade using palladium-containing powder of the present invention. Thepowder may include single-phase metallic particles and/or multi-phaseparticles. Furthermore, the metallic phase in the particles will oftenbe pure palladium, but more often includes one or more other metal aswell, such as silver, platinum and/or gold, preferably in an alloy withthe palladium. Particularly preferred is an alloy with silver, and moreparticularly silver-rich alloys. Preferably, the palladium-containingpowder for making interconnections in multi-layer ceramic packages, andin other circuit boards, will have a weight average particle size offrom about 0.5 micron to about 2 microns.

Furthermore, it will be appreciated that the surface layer ofmulti-layer ceramic packages will typically include metallized bondinglocations for the mounting of electronic components. These bondinglocations may include, for example, wire bond pads or solder pads. Thesemetallized bonding locations may also be made using palladium-containingpowder of the present invention.

Boards including interconnections made from powder of the presentinvention may be manufactured by any suitable process known in the art,and need not be significantly modified to accommodate the making ofinterconnections using powder of the present invention. Typically,multi-layer ceramic packages are made by sequential processing andstacking of ceramic green tapes. The green tapes are punched and formedto provide desired structural features. The metallized features are thenadded, such as by screen printing of conduction lines and filling ofvias by syringe or otherwise. The metallized ceramic layers are thenstacked to form a layered structure which is then cofired. Anyconvenient firing temperature may be used. Often, firing temperatures offrom about 300° C. to about 700° C. are used.

Patterned circuit lines for a variety of products other than circuitboards may also be made with palladium-containing powder of the presentinvention. For example, lines for antennas used in cellular telephonesmay be made using the palladium-containing powders. Another applicationfor patterned circuit lines are the resistive lines in surge resistors.Such resistors have serpentine meander circuit patterns of resistivelines for handling high current surges. The palladium-containing powderof the present invention may be mixed with powder of a resistivematerial, such as a metal ruthenate, to provide the proper resistivity.Also, the particles could include the resistive material in a secondphase in multi-phase palladium-containing particles. The palladium willtypically be a pure palladium metallic phase or as an alloy, preferablywith silver. Yet other applications for patterned circuit lines includeconductor lines on spiral, two-dimensional inductors and membraneswitches. In a membrane switch, the palladium-containing powder is usedto make conductive lines on a flexible membrane used to close the switchwhen depressed. Single or multi-phase particles may be used, and themetallic phase may be pure palladium or may include another metal,preferably silver or platinum, in an alloy with the palladium.

Another important application for palladium-containing powder of thepresent invention is in the manufacture of flat panel displays, andparticularly to make address electrodes for addressing a pixel area inthe display. The pixel area may be a whole pixel, such as wouldtypically be the case in a monochrome display, or the pixel area may bea subpixel of one component color of a whole pixel, such as wouldtypically be the case in a color display. Although the descriptionprovided here is exemplified primarily with a discussion of monochromeflat panel displays, the principles discussed apply equally to colorflat panel displays. Plasma flat panel displays are particularlypreferred for use of palladium-containing address electrodes made usingpowder of the present invention. For making address electrodes in flatpanel displays, the palladium-containing powder preferably has a weightaverage particle size of from about 1 micron to about 2.5 microns, morepreferably from about 1.5 microns to about 2 microns. The powder isoften of single phase metallic particles, although multi-phase particlesmay also be used. The metallic phase may include only palladium, but ispreferably in an alloy. A preferred alloy is a silver-rich alloy.

Referring now to FIG. 54, one embodiment of a flat panel display 520 isshown. The flat panel display 520 includes a plurality of displayelectrodes 522 and a plurality of address electrodes 524. The area ofoverlap between a display electrode 522 and an address electrode 524defines a pixel area 526. During operation of the display panel 520,when a voltage is applied between a display electrode 522 and thecorresponding address electrode 524, then the pixel area 526 to whichthe voltage is applied is energized to illuminate phosphor materiallocated in the pixel area 526.

With continued reference to FIG. 54, the display electrodes 522 areoften called front electrodes because they are on the front side of theflat bed display that faces the viewer during use. Therefore, thedisplay electrode is typically made of a transparent material, suchindium tin oxide. The address electrodes 524 are often called backelectrodes because they are on the side of the flat panel display 520that faces away from the viewer during use. The address electrodes 524are advantageously made using palladium-containing powder of the presentinvention.

One preferred flat panel display for manufacture usingpalladium-containing powder of the present invention are plasma flatpanel displays. Plasma flat panel displays illuminate through theactivation of phosphor materials by an ionic plasma, typically a plasmaof neon or another noble gas. Referring now to FIG. 55, one embodimentof a DC plasma flat panel display 530 is shown. The DC plasma flat paneldisplay 530 includes display electrodes 532 and address electrodes 534with a vapor space 535 being located between the display electrodes 532and the address electrodes 534. Inside the vapor space 535 is a gas,such as argon, which will form the ionic plasma when energized. The DCplasma flat panel display 530 also includes phosphor material 536 inpixel areas adjacent the display electrodes 532 and opposite the addresselectrodes 534. The address electrodes 534 are separated by dielectricribs 538. The display electrodes 532 and the phosphor material 536 aresupported on a transparent, dielectric front substrate 540, commonlyglass. The address electrodes 534 and the ribs 538 are supported on aback substrate 542, such as of glass or a ceramic material. The displayelectrodes 534 are made using palladium-containing powder of the presentinvention.

With continued reference to FIG. 55, when the DC plasma flat paneldisplay 530 is in operation, a circuit will be completed in an area ofoverlap between one of the display electrodes 532 and one of the addresselectrodes 534 corresponding with the pixel area in which phosphormaterial 536 is to be illuminated through the creation of a plasma inthe vapor space 535 corresponding with the pixel area. The ribs 538provide some protection against inadvertent illumination of adjacentpixel areas.

Referring now to FIG. 56, one embodiment of an AC plasma flat paneldisplay 550 is shown. The design is similar to that shown for the DCflat panel plasma display shown in FIG. 55 and the reference numerals inFIG. 56 refer to the same elements as discussed with respect to FIG. 55the DC plasma display 530, except as noted. As shown in FIG. 56, the ACplasma flat panel display 550 does not include ribs or other barrierpartitions to separate the address electrodes 534. The AC plasma displaypanel 550 does, however, include a front dielectric layer 552, whichprotects the display electrodes 532, and a back dielectric layer 554,which protects the address electrode layers 534. In color displayapplications, it would generally be advisable even in AC devices toinclude some type of barrier partition between the address electrodes534 to prevent inadvertent illumination of an adjacent sub-pixel of acolor that is not desired. Several configurations are known for both DCand AC devices and for monochrome and color devices, all of which arewithin the scope of the present invention. The examples shown in FIGS.55 and 56 are illustrative only.

Yet another application for the palladium-containing powder is as afiller material in polymer compositions to impart electrical or thermalconducting properties. One preferred composition is an epoxy adhesivefilled with the palladium-containing powder. The powder is preferably asilver-rich alloy with palladium and has a weight average particle sizeof from about 0.5 micron to about 3 microns.

Many of the applications just described include one or morepalladium-containing films made using palladium-containing powder of thepresent invention. The film may extend over a large area, such as wouldbe the case, for example, for internal electrodes of multi-layercapacitors and metallized terminations. Alternatively, the film may bein the form of a narrow line, or pattern of lines, such as would be thecase, for example, for conductive lines in multi-chip modules,serpentine resistor circuits, spiral inductors, antennas in cellulartelephones, window defoggers/deicers, photovoltaic grid electrodes, flatpanel display electrodes and membrane switches.

The palladium-containing films are typically made by a thick filmdeposition technique. Thick film deposition techniques generally involveapplication of a layer of a slurry, or paste, including thepalladium-containing powder on a substrate. The slurry is often in theform of a thick paste. The slurry may be applied by any technique, suchas by screen printing, doctor blading, dip coating, spray coating orother technique for applying a uniform layer. Screen printing isgenerally preferred for making thinner films and for making lines, oranother film pattern. The applied layer is eventually fired, or cured,at temperatures typically higher than about 300° C., to remove residualorganic components and to sinter/densify the material of thepalladium-containing particles to form a dense, palladium-containingfilm on the substrate. For some applications firing temperatures may be1000° C. or more. In some applications, the palladium-containing filmwill be cofired with other layers. For example, a multi-layer capacitoris manufactured by forming a stack of multiple alternating layers ofceramic substrate (often based on a titanate, such as barium titanate orneodymium titanate) and solid residue of the thick film paste. Theentire stack is then fired together in a single firing. Some processesuse high firing temperatures (generally above about 800° C.) and someprocesses use low firing temperatures (generally below about 700° C.).Also, the ceramic substrates for cofiring are most often low temperaturecofire ceramics.

The thick film slurry, or paste, typically includes the particles of thepalladium-containing powder dispersed in a liquid vehicle, which acts asa carrier liquid and is often referred to as a solvent. Pastecompositions are well known in the art and can have a reasonably complexchemistry, including solvents, binders and other additives to aid in thedispersion and flow properties of the paste. The palladium-containingpowder of the present invention may be substituted forpalladium-containing particles currently used in thick film pastecompositions, without significant modification of the paste formulation.Pastes manufactured with the palladium-containing powder of the presentinvention will, however, exhibit improved performance due to thesuperior properties of the powder, as discussed previously. In additionto palladium-containing powder, and a solvent, the pastes of the presentinvention typically include a binder, a thickener or resin, astabilizing agent and a wetting agent. The relative quantities ofbinders, thickeners, solvents, stabilizing agents and wetting agents areknown in the art and will vary depending upon the specific application.The binder can be, for example, a glass frit which controls thesintering characteristics of the film. The thickener imparts a desiredviscosity to the paste and also acts as a binding agent in the unfiredfilm. Examples of thickeners include ethyl cellulose and polyvinylacetates. The liquid carrier/solvent assists in mixing of the componentsinto a homogenous paste and evaporates rapidly upon application of thefilm. Usually the solvent is a volatile liquid such as methanol,ethanol, other alcohols or the like. The stabilizing agents preventoxidation and degradation, stabilize the viscosity or buffer the pH ofthe paste.

The palladium-containing powder of the present invention exhibits gooddispersibility of particles in a paste due to the narrow particle sizedistribution, a low degree of particle agglomeration and spheroidalparticle shape. Improved dispersion in the paste results in smootherprints, having fewer lump counts, and sharper print edges. Thedispersibility and flowability may be further improved, however, ifdesired. One method for improving the dispersibility is to include inthe particles a second material phase, as previously discussed, such asa metal oxide, that improves the dispersibility/flowability of theparticles in a paste. Also, palladium-containing particles canadvantageously be coated with an organic layer to provide improveddispersibility. The organic layer can advantageously be placed, forexample, onto a previously formed oxide coating overpalladium-containing metallic cores. For example, an appropriate organicgroup could be bonded to a silica coating, or to another oxide coating,through the use of a silane coupling agent. Examples of silanes thatcould be used as such a coupling agent include halo, amido and alkoxysilanes.

The use of the palladium-containing powder of the present invention forthick film applications is particularly preferred for these thick filmapplications because of ability of the powder to make a high performancethick film with the use of a smaller quantity of palladium than withpalladium-containing powders currently used in thick film applications.This is because of the extremely high quality of the powder of thepresent invention, as previously described. This result is even moresurprising when it is considered that the powder of the presentinvention can often be made less expensively than powders in currentuse, resulting in significant cost savings for applications using largequantities of palladium. This significant cost savings is particularlysurprising considering the higher performance characteristics of thepowder.

One significant performance advantage of the palladium-containing powderof the present invention is that it can be used to make a very thinthick film with highly definable edges. This is particularly importantwhen reducing device thickness, such as with multi-layer capacitors, orproviding an increased density of conductive lines, such as inmulti-chip modules, is desirable. In that regard, the thick films madewith the present invention, after firing, may be made with a thicknessof smaller than about 10 microns, preferably smaller than about 8microns, more preferably smaller than about 6 microns, and mostpreferably smaller than about 4 microns.

Furthermore, when making thick film electrically conductive lines, thelines may be made with a sharp edge definition, due to thecharacteristics of the powder. Lines of a narrow width may be made andwith a very close pitch. Lines may be made with a width of smaller thanabout 50 microns, preferably smaller than about 25 microns, and morepreferably smaller than about 15 microns. Line pitch may be smaller thanabout 100 microns, more preferably smaller than about 50 microns, andmost preferably smaller than about 30 microns. The line pitch is thecenter-to-center spacing of the lines.

To make extremely narrow lines with a small pitch, it is frequentlydesirable to first deposit lines by thick film techniques and then trimthe lines for better edge definition. Trimming may be accomplished byknown methods, such as for example laser trimming. A preferred methodfor obtaining the desired edge definition, however, is to use aphotolithographic technique, such as the FODEL™ process of DuPont. Forexample, the powder may be mixed with a photocurable polymer to permitphotolithographic patterning using a mask. Undeveloped areas are removedby use of a solvent. The remaining polymer is then removed when the filmis fired.

Another major advantage of the palladium-containing powders of thepresent invention is that they can be made substantially free of organiccontaminants, such as surfactants, which are a problem with powders madeby liquid precipitation. Because of the absence of such organiccontaminants in the particles of the powder, a conductive film of veryhigh conductivity may be prepared, even when the film is in the form ofa thin, narrow conductive line as previously described. Preferably, whenmaximum conductivity is desired, the film made usingpalladium-containing powder of the present invention has an electricalconductivity of at least about 80%, more preferably at least about 85%,and most preferably at least about 90% of the bulk electricalconductivity of the metallic phase of the film, which may be of purepalladium, but is more often an alloy of palladium with one or moreother metal. A preferred alloy is with silver and particularly asilver-rich alloy. Such high conductivity lines are particularlyimportant in some devices, such as, for example, cellular telephones andother high frequency applications.

Another significant advantage of using the palladium-containing powderof the present invention is that multi-phase particles may be used toalter densification, sintering and other characteristics for improvedcompatibility with another layer. For example, sintering of apalladium-containing metallic phase in the powders may be delayed,and/or adhesion to a substrate improved, by incorporation of one or moreother phases. This is particularly advantageous for metallic phases of apalladium/silver alloy, and especially for silver-rich alloys. Forexample, the powder may include multi-phase particles having an intimatemixture of a palladium/silver metallic phase and a second phaseincluding silica or another ceramic material. Also, the particles mayinclude a surface coating of a material, such as silica, that delayssintering of a core of a palladium/silver metallic phase and alsoprovides enhanced adhesion to a substrate. As one example, the topdielectric layer of multi-chip modules is often a glass layer. Usingpure silver for conductive lines or other features adjoining the glasslayer is problematic because at the high sintering temperature of theglass, the silver is highly mobile and will diffuse into the glass. Thisproblem can be somewhat reduced by mixing in some palladium particleswith the silver particles, but silver migration is still a problem. Themobility of silver can be significantly reduced, however, if the silveris alloyed with palladium, but the alloy generally does not adhere wellto the glass dielectric, which could cause delaminations to occur. Withthe powder of the present invention, the palladium/silver alloy could bein multi-phase particles having a coating or intimate mixture of silicaor another adhesion promoting material that also delays sintering of thesilver to reduce mobility of silver.

One major advantage of using palladium-containing powder of the presentinvention to manufacture electronic devices, and especially multi-layerdevices such as multi-layer capacitors and multi-layer ceramic packages,is that the palladium may be incorporated into the powder in a manner toprovide a high resistance of palladium to oxidation. For example, theresistance of the particles to oxidation of the palladium issignificantly enhanced with the multi-phase particles as describedpreviously. Improving the resistance of particles to oxidation of thepalladium is very important for many applications in which thepalladium-containing powder is to be used to form an electricallyconductive film for electronic components. This is because duringmanufacture of such electronic components, palladium often goes throughan oxidation and reduction cycle during firing of the film, whenprocessing temperatures may reach as high as from about 600° C. to 900°C., or more. This is problematic because there is a significant volumeexpansion upon the conversion of palladium to palladium oxide and acorresponding volume contraction with reduction from palladium oxideback to palladium. Volumetric changes that can occur during processingcan result in film delaminations and cracking.

One method that has been proposed by others for reducing thesusceptibility of palladium to oxidation in particles is to add a smallquantity, preferably from 0.005 percent to 0.1 percent by weightrelative to palladium, of one or more alkaline earth metal elements.(See, U.S. Pat. No. 5,402,305 by Asada issued Mar. 28, 1995.) Oxidationresistance of palladium in the particles of the present invention mayalso often be improved by the addition of an alkaline earth metal. Withthe present invention, however, other additives could be used in smallquantities, instead of an alkaline earth metal, for the purpose ofimproving palladium oxidation resistance.

One additive that has been found to be particularly effective forreducing the susceptibility of palladium to oxidation in the particlesof the present invention is tin. Tin may be added in the form of asoluble precursor in the liquid feed 102 when preparing particles.Examples of soluble precursors include tin chlorides, nitrates,acetates, oxalates and the like. Preferably, the tin should be added inquantities of from about 0.005 weight percent to about 1 weight percentrelative to the palladium, with quantities of from about 0.05 weightpercent to about 0.5 weight percent being preferred.

Another method for reducing problems associated with volumetric changesdue to oxidation of palladium is to include the palladium in a highquality alloy with another metal. A preferred alloy metal is silver,because silver has good electrical conductivity and is not verysusceptible to oxidation during processes for manufacturing electroniccomponents. The amount of volumetric expansion due to palladiumoxidation will then be reduced by at least the amount of materialoccupied by the silver, plus beneficial effects provided by alloying. Inthat regard, however, it has been found that to prepare a powder ofparticles with a true alloy between palladium and silver, i.e.,homogeneous at the molecular level, it is very important to carefullycontrol conditions in the furnace during manufacture. Although notwishing to be bound by theory, it is believed that the careful controlof the furnace is required due to the relatively high vapor pressure ofsilver at the elevated temperatures often encountered during spraypyrolysis.

The particular alloy composition will depend upon the specificapplication for which the particles are intended. For most applications,however, palladium/silver alloys of the present invention preferablyinclude from about 10 weight percent to about 70 weight percentpalladium and from about 30 weight percent to about 90 weight percentsilver.

For electronic components manufactured in low fire processes (withfiring temperatures of less than about 700° C.), silver rich alloys arepreferred. Preferred alloys for low fire processes include from about 10weight percent to about 30 weight percent palladium and from about 70weight percent to about 90 weight percent silver. Particularly preferredfor low fire processes are alloys having a weight ratio of Pd:Ag ofabout 20:80 and 30:70.

For electronic components manufactured in high fire processes (withfiring temperatures above about 800° C.), palladium rich alloys arepreferred. Preferred alloys for high fire processes include from about50 weight percent to about 70 weight percent palladium and from about 30weight percent to about 50 weight percent silver. Particularly preferredfor high fire processes are alloys having a weight ratio of Pd:Ag ofabout 50:50 and 70:30.

According to the present invention, it has, surprisingly, been foundthat certain spray pyrolysis manufacturing conditions are particularlyconducive to preparation of a true alloy between palladium and silverand that such processing conditions result in the manufacture ofpalladium-containing particles that have a significantly enhancedresistance to oxidation, even for pure palladium particles. With properprocessing, the susceptibility of palladium to oxidize may be reducedsuch that less than about 40 percent of the palladium is susceptible tooxidation, determined based on maximum weight gain duringthermogravimetric analysis (TGA), assuming all weight gain isattributable to oxidation of palladium, with the percentage beingrelative to a theoretical weight gain for oxidation of all palladium inthe particles. Preferably, less than about 35 percent, more preferablyless than about 30 percent, even more preferably less than about 25percent, and most preferably less than about 20 percent of the palladiumis susceptible to oxidation. These weight gains are based on TGA atatmospheric pressure using industrial grade air and with temperaturesincreasing up to about 900° C., or more, at a temperature rise of about10° C. per minute. The starting temperature is typically roomtemperature, but may be any convenient temperature below about 400° C.As used herein, industrial grade air refers to generally commerciallyavailable grade of liquefied air of a high purity, preferably at least99.9% pure.

According to the process of the present invention, when making particlesincluding a palladium/silver alloy, or for making pure palladium, thefurnace temperature should be in a range with a lower limit of about900° C., preferably about 950° C., more preferably about 975° C., andeven more preferably about 1000° C.; and with an upper limit of about1300° C., preferably about 1200° C., more preferably about 1150° C.,even more preferably about 1100° C., and still more preferably about1050° C. If the temperature is too low, the particles do not adequatelydensify and significant porosity in the particles can result insignificant susceptibility of the palladium to oxidation. As thetemperature exceeds about 1100° C. the vapor pressure of silver becomessignificant, and significant vaporization of silver during particlemanufacture appears to occur, which can result in the production ofundesirable ultrafines of silver. This problem becomes particularlypronounced above about 1200° C. Such vaporization can result insignificant segregation of silver and defeat the objective of preparinga true alloy between all of the palladium and the silver in the feed.The presence of a segregated, unalloyed silver phase in a powder isundesirable because of the high mobility of silver in microelectronicdevices when the silver is in an unalloyed state. Furthermore, theapparent low degree of alloying between palladium and silver at highermanufacture temperatures results in an increased susceptibility ofpalladium to oxidation relative to the preferred processing temperaturerange.

For example, when making alloys of palladium and silver, or when makinga substantially pure palladium metallic phase, maximum average streamtemperatures in the furnace should preferably be in a range of fromabout 950° C. to about 1100° C. More preferred is a temperature range offrom about 975° C. to about 1050° C., with a manufacture temperature ofapproximately 1000° C. being particularly preferred. These temperaturesare preferred for both palladium-rich and silver-rich alloys.

Furthermore, it has been found that the vapor pressure of palladium, andother metals such as silver in the case of an alloy, should bemaintained at a relatively low level during processing to avoid theproduction of ultrafines that degrade the quality of the particulateproduct and are highly detrimental to resistance of the palladium tooxidation. When the maximum average stream temperature in the furnace ismaintained at a temperature low enough to prevent the vapor pressure ofone or all of the metals from exceeding 100 millitorr, and preferably 50millitorr, it has been mostly surprisingly found that the resultingparticles are generally of a higher quality and generally have a lowerresistance to oxidation of palladium. This is especially important whenthe palladium is alloyed with silver.

Even more surprising is that the palladium-containing particles of thepresent invention exhibit exceptional resistance to oxidation eventhough the palladium-containing metallic phase of particles typically ispolycrystalline and even when the particles are substantially free ofadditives, such as alkaline earth metals. It could be expected thatpolycrystalline particles would exhibit poor oxidation resistance due tomigration of oxygen into the interior of the particles along grainboundaries. Surprisingly, however, polycrystalline palladium-containingparticles of the present invention are found to have excellentresistance to palladium oxidation.

The ability to produce polycrystalline palladium-containing particleswith good oxidation resistance is of commercial significance because theprocessing expense of producing substantially single crystal particlesis avoided. Processing to obtain single crystal particles would normallyrequire either that the particles be manufactured at a temperature thatis near to or higher than the melting temperature of the palladium, orthe palladium-containing alloy, or that the particles be subjected to alengthy annealing operation requiring long furnace residence times. Theparticles of the present invention, however, are manufacturable attemperatures significantly below the melting temperature and do notrequire a lengthy anneal, saving a significant amount of energy andpermitting short residence times in the furnace during manufacture. Evenwhen the palladium-containing metallic phase is polycrystalline,however, the mean crystallite size is still relatively large, normallylarger than about 50 nanometers and preferably larger than about 100nanometers.

A further consideration in the manufacture of palladium-containingparticles to reduce the susceptibility of palladium to oxidation is thecarrier gas used to prepare the aerosol from which the particles aremanufactured and the quench gas used to cool the particles. It has beenfound that the use of an inert gas, such as nitrogen, is preferred tothe use of air or other carrier gases including oxygen gas. An inertgas, such as nitrogen, may also be used as the quench gas.

EXAMPLES

The following examples are provided to aid in understanding of thepresent invention, and are not intended to in any way limit the scope ofthe present invention.

Example 1

This example demonstrates manufacture of single-phase palladiumparticles that include a small amount of calcium to improve resistanceto oxidation of the palladium.

An aqueous solution including 5 weight percent palladium in the form ofa dissolved nitrate and 0.26 weight percent calcium, relative to thepalladium, in the form of a dissolved carbonate. In one case, an aerosolis generated using an ultrasonic generator including 6 ultrasonictransducers operating at 1.6 MHz. In another case, an aerosol isgenerated with an ultrasonic generator including an array of 49ultrasonic transducers operating at 1.67 MHz. Both aerosols are thensent to a furnace at a temperature of 1100° C., where the palladiumparticles are produced by spray pyrolysis of the aerosol. Nitrogen gasis used as a carrier gas for the aerosol in both cases. Nitrogen is usedas the quench gas in the case of the 6 transducer generator and air isused as the quench gas in the case of the 49 transducer generator. Inthe case of the 49 transducer generator, the aerosol is classified usingan impactor upstream of the furnace reactor.

The particles in both cases are dense, spheroidal and of high quality.The particles for both cases disperse well in paste compositions.

Example 2

This example demonstrates the preparation of palladium-containing powderin which the palladium is present in a 70/30 palladium/silver alloy.

An aqueous solution is prepared including dissolved palladium and silveras nitrates. The total amount of palladium and silver in the solution is5 weight percent, with the relative proportions of palladium/silverbeing 70/30 on a weight basis, so that, if the silver and the palladiumfully alloy, a 70/30 Pd/Ag alloy will be obtained in the particles. Anaerosol is generated from a single transducer ultrasonic generatoroperating at a frequency of 1.6 MHz using a carrier gas of nitrogen.Generated aerosols are sent to a furnace to prepare thepalladium-containing particles. Furnace temperatures are varied from900° C. to 1400° C. The samples are cooled and collected.

Several particle samples are subjected to TGA testing in air to evaluatethe weight gain of the particles as an indication of susceptibility topalladium oxidation. Also, several particle samples are subjected toatomic absorption spectroscopy (AAS) to estimate the alloy composition.TGA and AAS results are shown in Table 1. As seen in Table 1, asignificant amount of silver does not alloy with the palladium in theparticles manufactured at higher temperatures. Also, from the TGAinformation, oxidation resistance is very high for particlesmanufactured at 1000° C. and 1100° C. cases, and is particularly goodfor the 1000° C. case. For example, for the particles manufactured at1000° C., only about 13% of the palladium appears to be susceptible tooxidation, based on the TGA, assuming that all weight gain during theTGA is attributable to palladium oxidation. TABLE 1 Example 2 70/30Pd/Ag Feed Est. Alloy Reactor Temp. TGA Max. Weight Gain Composition⁽¹⁾(° C.) (% of original Pd wt.) (Wt. % Ag) 900 30 27.6 1000 13 1100 2521.3 1400 33 15.9⁽¹⁾Atomic absorption spectroscopy

Referring now to FIG. 57, an SEM photomicrograph is shown of theparticles produced at 900° C. As seen in FIG. 57, there is significantporosity that appears in the particles, accounting for the lowresistance to palladium oxidation indicated by the TGA results at thattemperature. FIG. 58 is an SEM photomicrograph showing particlesprepared at 1000° C. These particles are significantly more dense and,accordingly, do not exhibit the same susceptibility to oxidation ofpalladium as samples at 900° C. As the temperature of manufactureincreases, a significant amount of ultra-fine silver particles is found.These ultra-fine silver particles (typically 30-50 nanometers in size)are believed to be formed from the significant quantities of silver thatvaporize at higher manufacturing temperatures. This accounts for the lowlevels of silver found in the palladium/silver alloys, as previouslynoted for higher manufacturing temperatures. The presence of theseultra-fine particles can be seen in FIG. 59, which shows an SEMphotomicrograph of particles produced at 1400° C. Generally, the loss ofsilver from the alloy increases with increasing temperature, especiallyas the temperature approaches the melting range for the desired alloy.In that regard, the melting range for a 70/30 palladium/silver alloy isabout 1374.5° C. to about 1431.5° C.

Example 3

This example demonstrates the detrimental effect on palladium oxidationresistance of using air as a carrier gas in the manufacture of particleswith a palladium/silver alloy.

Palladium/silver alloy particles are made according to the procedure ofExample 2, including a 70/30 Pd/Ag weight ratio in the liquid feed,except that air is used as the carrier gas instead of nitrogen. Theparticles are manufactured with a furnace temperature of 1000° C.

Based on TGA testing of the particles in air, assuming all weight gainis attributable to palladium oxidation, only about 26 percent of thepalladium oxidizes, further demonstrating the beneficial effect offurnace temperature on oxidation resistance. Even though acceptable formany applications, this level of palladium oxidation is about twice aslarge as shown in Example 2 when nitrogen is used as the carrier gas.

Example 4

This example demonstrates the addition of calcium to a 70/30 Pd/Agalloy.

Particles are prepared according to the procedure of Example 2, exceptthat 0.25 weight percent calcium relative to palladium is added innitrate form to the liquid feed. Particles are produced at a furnacetemperature of 1000° C. TGA indicates that about 18 percent of thepalladium in the particles is susceptible to oxidation, indicating thatthe calcium addition has not improved oxidation resistance relative toprocessing at 1000° C. in the absence of calcium, as shown in Example 3.This result is particularly surprising considering the teachings of U.S.Pat. No. 5,402,305 by Asada describing the beneficial effects of addingcalcium to palladium powders.

Example 5

This example demonstrates preparation of powder of a 30/70 Pd/Ag alloy.

Particles are prepared according to the procedure of Example 2, exceptthat palladium and silver in the feed solution are in a weight ratio of30/70 Pd/Ag. Particles are prepared at furnace temperatures of 900° C.,1000° C., 1100° C., 1225° C., 1300° C., 1400° C., and 1500° C.

Similar to the results obtained for 70/30 Pd/Ag tests, the 30/70 Pd/Agtest results show that appreciable amounts of silver vaporize duringmanufacture at higher furnace temperature. This segregation of silver isparticularly detrimental for microelectronic thick film applications,because the unalloyed silver is significantly more mobile than alloyedsilver and can significantly impair the operation of manymicroelectronic devices. Furthermore, for particles preparedsignificantly below 1000° C., the particles exhibit significantporosity, which is undesirable. Temperatures of from about 900° C. to1200° C. are preferred, with temperatures of from about 950° C. to 1100°C. being more preferred for 30/70 Pd/Ag compositions.

Example 6

This example demonstrates the preparation of particles including analloy of palladium and nickel.

An aqueous solution is prepared including nickel and palladium asdissolved nitrates. The total of the metals in the mixture is 5 weightpercent, with the nickel and palladium being proportioned 30/70 Pd/Ni byweight. This solution also includes 0.3 weight percent or tin, as adissolved nitrate, relative to the total weight of palladium and nickel.An aerosol of the solution is produced via a single transducerultrasonic generator operating at 1.6 MHz and is sent to a furnace at atemperature of 1000° C. where particles are produced. The aerosolcarrier gas comprises 95 percent nitrogen by volume and 5 percenthydrogen by volume.

X-ray defraction measurements of the powder produced indicates a truenickel-palladium alloy with substantially no segregated nickel orpalladium. TGA measurements indicate significant resistance of theparticles to oxidation.

Example 7

This example demonstrates preparation of multi-phase particles includingpalladium and silica.

A feed liquid is prepared by dissolving 60 nanometer silica spheres in a2.2 weight percent aqueous palladium nitrate solution in an amount toprovide 25 weight percent silica in the palladium-containing particles.The liquid feed is converted to an aerosol in a single transducerultrasonic generator at a frequency of 1.6 MHz using nitrogen as acarrier gas. Particles are prepared at a variety of furnace temperaturesranging from 900° C. to 1100° C.

Particles prepared at 900° C. were found to include a morphology ofintimately mixed phases of palladium and silica. With increased reactortemperature, however, the particles tended to segregate and particlesprepared at 1100° C. had the morphology of a silica coating over apalladium core.

Example 8

This example further demonstrates preparation of multi-phase particlesincluding palladium and silica.

Silica spheres of the size of approximately 200 nanometers are suspendedin a 2.2 weight percent palladium nitrate aqueous solution in an amountto provide 25 weight percent silica in the palladium-containingparticles. Using nitrogen as a carrier gas, an aerosol of the liquidfeed is produced in a single transducer ultrasonic generator at afrequency of 1.6 MHz. The aerosol is sent to a furnace at a temperatureof 1100° C. to produce the palladium-containing multi-phase particles.The particles are cooled and collected.

The particles are found to be a mixture of particles including palladiumdomains dispersed on silica cores and silica cores coated with thepalladium. Higher temperatures would promote a more uniform coating ofthe silica cores.

Example 9

This example further demonstrates preparation of multi-phase particlesincluding palladium and silica.

A liquid feed is prepared by suspending colloidal silica in a 2.2 weightpercent aqueous solution of palladium nitrate. The liquid feed isconverted to an aerosol using nitrogen as a carrier gas in a singletransducer ultrasonic generator at a frequency of 1.6 MHz. The furnaceis at a temperature of 1100° C. Particles exiting the furnace are cooledand collected. Concentrations in the feed range from 75/25 Pd/SiO₂ to54/46 Pd/SiO₂ by weight.

For all the compositions, the particles have a morphology of an intimatemixture of silica and palladium. An SEM photomicrograph for particlesincluding 45 weight percent silica and 55 weight percent palladium isshown in FIG. 60.

Example 10

This example demonstrates preparation of multi-phase particles includingpalladium and barium titanate.

A liquid feed is prepared including dissolved barium nitrate andtitanium tetraisopropoxide dissolved in a 2.2 weight percent aqueouspalladium nitrate solution. An aerosol of the liquid feed is prepared ina single transducer ultrasonic generator at a frequency of 1.6 MHz usingnitrogen as a carrier gas. The aerosol is converted to particles in afurnace at a temperature of 1100° C. The particles are collected andcooled. The amounts of barium nitrate and titanium isopropoxide arevaried to result in particles of varying composition between 25 and 45weight percent barium titanate.

For all tested compositions, the particles are found to include a bariumtitanate phase mixed with a palladium phase. Such multi-phase particlescould be particularly useful as an interlayer, or in an electricallyconductive layer, to improve compatibility between a barium titanateceramic substrate and an electrically conductive palladium-containingfilm.

Example 11

This example demonstrates preparation of multi-phase particles of bariumtitanate with a 30/70 Pd/Ag alloy.

A barium titanate precursor solution is prepared by dissolving 2.8 gramsof barium nitrate in 50 milliliters of titanium tetraisopropoxide, withrapid stirring, and finally adding 2 milliliters of concentrated nitricacid.

A Pd/Ag alloy precursor solution is prepared with 2.5 weight percentpalladium and silver in a weight ratio of Pd:Ag of 30:70, with thepalladium and silver in the form of dissolved nitrates.

Various mixtures are prepared of the barium titanate precursor solutionand the Pd/Ag alloy solution for preparation of particles with differentrelative quantities of the alloy and barium titanate. Compositionsinclude those with from 10 weight percent to 90 weight percent bariumtitanate in 10 weight percent increments and also a compositionincluding only 5 weight percent barium titanate.

Aerosols are prepared using an ultrasonic aerosol generator at afrequency of 1.6 MHz including a single ultrasonic transducer and withan impactor prior to entry of the aerosol into a furnace reactor wherethe particles are made. The particles are characterized by powder x-raydiffraction, scanning electron microscopy (SEM), energy dispersivespectroscopy (EOS), thermogravimetric analysis (TGA) and heliumpycnometry (for density). Maximum average stream temperatures in thefurnace are varied from 600° C. to 1100° C. Air is used as a carriergas.

All of the particles are dense, spheroidal and include a true compositeof the alloy and barium titanate phases. Generally, particle density, asa percentage of theoretical, decreases with increasing furnacetemperatures.

FIG. 61 shows an SEM photomicrograph of a composite particle including20 weight percent barium titanate made at 1000° C.

FIG. 62 shows a TEM photomicrograph of composite particles including 20weight percent barium titanate, made at a furnace temperature of 1000°C., showing areas indicated by EDS to be rich in the Pd/Ag alloy andareas rich in the barium titanate. FIG. 63 shows a TEM photomicrographof composite particles including 5 weight percent barium titanate madeat a furnace temperature of 1000° C., showing areas indicated by EDS tobe rich in the Pd/Ag alloy and rich in the barium titanate.

Example 12

This example demonstrates preparation of multi-phase particles includingpalladium and titania.

A liquid feed is prepared including titanium tetraisopropoxide dissolvedin a 2.2 weight percent aqueous solution of palladium nitrate. Theliquid feed is converted to an aerosol in a single transducer ultrasonicgenerator at 1.6 MHz using nitrogen as a carrier gas. The aerosol isconverted to particles in a furnace at a temperature of 1100° C.Particles are collected and cooled. The amount of titaniumtetraisopropoxide in the liquid feed is varied to produce from about 25weight percent to about 45 weight percent of titanium oxide in the finalparticles.

The particles for all tested compositions show the palladium metal beingdispersed on a high surface area titania support. Such particles may beused for various catalyst applications.

Example 13

This example demonstrates the preparation of multi-phase particlesincluding palladium and alumina.

A feed liquid is prepared including aluminum sec-butoxide dissolved in a2.2 weight percent aqueous palladium nitrate solution. A liquid feed isconverted to an aerosol in a single transducer ultrasonic generator at afrequency of 1.6 MHz using nitrogen as a carrier gas. The aerosol isconverted to particles in a furnace at a temperature of 1100° C. Theparticles are collected and cooled. The amount of the aluminumsec-butoxide is varied to provide from about 25 weight percent to about45 weight percent of alumina in the final particles.

Particles for all of the tested compositions were found to include thepalladium dispersed on a high surface area alumina support. Theparticles appear to be hollow. Such particles may be used for variouscatalyst applications.

Example 14

This example demonstrates preparation of multi-phase particles includingpalladium and titania, made from a feed liquid including suspendedtitania particles.

A liquid feed is prepared including 0.25 micrometer titania particlessuspended in a 2.2 weight percent aqueous palladium nitrate solution.Using a single transducer ultrasonic generator at a frequency of 1.6 MHzand using nitrogen as a carrier gas. The aerosol is converted toparticles in a furnace at a temperature of 1100° C. The particles arecooled and collected. The amount of titania in the liquid feed is variedto provide from 15 to 25 weight titania in the final particles.

The particles show a palladium coating on top of titania cores.

Example 15

This example demonstrates preparation of multi-phase particles by CVDcoating of palladium precursor particles.

An aqueous solution is prepared including palladium in the form of anitrate. An aerosol of the solution is generated in a single transducerultrasonic generator at a frequency of 1.6 MHz using nitrogen as thecarrier gas. Palladium precursor particles are prepared from the aerosolat furnace temperatures ranging from 1100° C. to 1300° C. Thepalladium-containing precursor particles exit the furnace in an aerosolform and enter a CVD coating apparatus in which a coating of eithersilica or titania is applied to the palladium precursor particles by CVDfrom either silica tetrachloride or titanium tetrachloride, as the casemay be.

All particles exhibited a coating of the desired silica or titania onpalladium cores.

Example 16

This example demonstrates the oxidation resistance exhibited by a numberof different particles made according to the present invention.

A number of composite particles are made including palladium in onephase and another material in a second phase. The composite particlesare tested by TGA to determine susceptibility of the particles topalladium oxidation. Table 2 identifies the composite material,conditions of particle manufacture, and the calculated weight percent ofpalladium oxides during TGA testing, assuming that all weight gainduring the TGA is attributable to palladium oxidation. As seen in Table2, many of the composite materials significantly decrease thesusceptibility of palladium to oxidation. In the case of silicacomposites, over 70 percent of the palladium appears to be notsusceptible to oxidation. TABLE 2 Example 15 Composite Mtl. Furnace Wt.% Pd Composite Material Concentration Temp. Oxidation/TGA SiO₂ 60 nmParticles 25 Wt. %  900° C. 100% SiO₂ 60 nm Particles 25 Wt. % 1000° C.28% SiO₂ 200 nm Particles 25 Wt. % 1100° C. 41% SiO₂ Colloid 25 Wt. %1100° C. 35% Particles SiO₂ Colloid 35 Wt. % 1100° C. 31% Particles SiO₂Colloid 45 Wt. % 1100° C. 27% Particles Al₂O₃ from Al(NO₃)₃ 25 Wt. %1100° C. 66% Al₂O₃ from Al(NO₃)₃ 35 Wt. % 1100° C. 49% Al₂O₃ fromAl(NO₃)₃ 45 Wt. % 1100° C. TiO₂ from Soluble 25 Wt. % 1100° C. 36%Precursor TiO₂ from Soluble 35 Wt. % 1100° C. 37% Precursor TiO₂ fromSoluble 45 Wt. % 1100° C. 38% Precursor TiO₂ from Particle 15 Wt. %1100° C. 53% Precursor TiO₂ from Particle 25 Wt. % 1100° C. 58%Precursor BaTiO₃ from Soluble 25 Wt. % 1100° C. 58% Precurs. BaTiO₃ fromSoluble 35 Wt. % 1100° C. 63% Precurs. BaTiO₃ from Soluble 45 Wt. %1100° C. 72% Precurs.

Example 17

This example demonstrates preparation of palladium-containing particlesincluding tin as a dopant to enhance oxidation resistance of palladiumin the particles.

An aqueous solution is prepared including 5 weight percent of palladiumas dissolved palladium nitrate. The solution is divided into severalbatches and to some batches a soluble tin precursor is added to achieve0.25 weight percent of tin in the solution relative to palladium. Twodifferent tin precursors are used. SnCl₂.2H₂O is used in some tests andSnCl₄.5H₂O is used in other tests. Solutions are aerosolized viaultrasonic nebulization at a frequency of 1.6 MHz using nitrogen as thecarrier gas. The aerosol is sent to a hot-wall tubular furnace. Testswere run at furnace temperatures of 900° C., 1100° C., 1300° C., and1500° C. Powders exiting the furnace were collected on a 142 millimeterdiameter Tuffryn filter (pore size 0.45 micron) supported in a Gelmanstainless-steel filter holder. The filter housing is heated to between50° C. and 70° C. to prevent water from condensing on the filter.

Powders are examined by x-ray defraction to determine the crystallinephase, thermogravimetric analysis (TGA) to examine resistance of thepowders to oxidation of palladium and scanning electron microscopy tostudy the morphology, size and size distribution of particles in thepowders. The TGA studies are performed over a range of 390° C. to 900°C. at a heating rate of 10° C. per minute in an atmosphere of industrialgrade air.

Results of thermogravimetric analysis for several tests is shown inTable 3. TABLE 3 Pure Pd Tin-doped Pd Reactor Temp. % Pd % Pd ° C.Oxidation Oxidation 900 33.7 26.5 1100 31.4 24.8 1200 36.1 30.0 150052.3 45.8

The data in Table 3 shows information for pure palladium particles andpalladium including tin doping provided by SnCl₂.2H₂O as a precursor.Table 3 shows the percent of palladium that oxidizes during the TGA,assuming that all weight gain during the TGA is due to oxidation ofpalladium, with the percentage being relative to a theoretical weightgain for complete oxidation of all palladium in the particles. As seenin Table 3, the tin addition provided a significant reduction in theamount of palladium oxidizing during the thermogravimetric analysis.

While various specific embodiments of the process of the presentinvention and the apparatus of the present invention for preparingsilver-containing particles are described in detail, it should berecognized that the features described with respect to each embodimentmay be combined, in any combination, with features described in anyother embodiment, to the extent that the features are compatible. Forexample, any or all of the aerosol concentrator, aerosol classifier,particle cooler, particle coater, particle modifier and addition of drygas may be incorporated into the apparatus and/or process of the presentinvention. Also, additional apparatus and/or process steps may beincorporated to the extent they do not substantially interfere withoperation of the process of the present invention or the apparatususeful therefore. For example, to further control the size distributionof particles produced accordingly to the process of the presentinvention, a particle classifier could be used after particle coolingand before particle collection. Other modifications will become apparentto those skilled in the art. All such modifications are intended to bewithin the scope of the present invention.

Also, while various embodiments of the present invention have beendescribed in detail, it is apparent that modifications and adaptationsof those embodiments will occur to those skilled in the art. It is to beexpressly understood, however, that such modifications and adaptationsare within the scope of the present invention, as set forth in theclaims below. Further, it should be recognized that any feature of anyembodiment disclosed herein can be combined with any other feature ofany other embodiment in any combination.

1-153. (canceled)
 154. An electronic device including apalladium-containing layer adjacent to a ceramic layer, the devicecomprising: a first layer including palladium adjacent to a second layerincluding a ceramic material and being substantially in the absence ofpalladium, said first layer being electrically interconnected with saidsecond layer when said electronic device is operational in a electricalcircuit; wherein, said first layer further comprising said ceramicmaterial and said first layer having been prepared from particlesincluding multi-phase particles having a weight average size of fromabout 0.1 micron to about 4 microns and with a size distribution suchthat greater than about 90 weight percent of said multi-phase particlesare smaller than twice said weight average size, with said multi-phaseparticles having a first material phase including said palladium and asecond material phase including said ceramic material.
 155. Theelectronic device of claim 154, wherein: said ceramic material is atitanate.
 156. The electronic device of claim 154, wherein: said ceramicmaterial is a titanate of at least one of barium, neodymium, calcium,magnesium, strontium and lead.
 157. The electronic device of claim 154,wherein: said electronic device comprises a capacitor including saidfirst layer as an electrically conductive layer and said second layer asa corresponding dielectric layer.
 158. The electronic device of claim154, wherein: said first layer comprises less than about 10 weightpercent of said ceramic material.
 159. The electronic device of claim154, wherein: said first layer comprises less than about 5 weightpercent of said ceramic material.
 160. The electronic device of claim154, wherein: electrical contact between said first layer and saidsecond layer being enhanced in comparison to electrical contact if saidfirst layer had been made with a mixture of particles consistingessentially of first particles substantially entirely of only said firstmaterial phase and second particles substantially entirely of only saidsecond material phase.
 161. A multi-layer capacitor includingpalladium-containing electrically conductive layers, the capacitorcomprising a structure having stacked layers including a plurality ofdielectric layers each including a dielectric material, with eachdielectric layer being adjacent to and electrically interconnected withat least one of a plurality of electrically conductive layers includingpalladium; electrical contacts interconnected with said electricallyconductive layers and said dielectric layers, said electrical contactsfor connecting the capacitor in an electrical circuit when the capacitoris used; said electrically conductive layers including said palladiumand at least some of said dielectric material; wherein, at least aportion of said palladium and said dielectric material in saidelectrically conductive layers being from multi-phase particles having aweight average size of from about 0.1 micron to about 4 microns andhaving a first material phase including said palladium and a secondmaterial phase including said dielectric material.
 162. The electroniccapacitor of claim 161, wherein: said first material phase comprises anelectrically conductive alloy including palladium and a second metal.163. The electronic capacitor of claim 162, wherein: said alloycomprises from about 10 weight percent to about 70 weight percentpalladium and from about 30 weight percent to about 90 weight percentsilver. 164-167. (canceled)